Add helper functions to create US standard atmosphere.

One of the standard atmosphere models is the US Standard Atmosphere - this is what is implemented in AIRES, and was copied by hand into the vertical_EAS example without the name (shown below) and has since been copied into almost every other example (lots of code duplication). This MR implements a helper function that can be used US Standard atmospheres quickly.

It also implements an implements_mixin utility that can be used to check whether TEnvironmentInterface satisfies one of the media mixins (i.e. refractivity, magnetic fields, etc.) - I didn't see any existing check for this but let me know if I missed it. This is used to optionally add the US Standard refractivity if the interface calls for a refractive index.

This code turns the following code that is used in every example:

auto builder = make_layered_spherical_atmosphere_builder<
      setup::EnvironmentInterface, MyExtraEnv>::create(center,
                                                       constants::EarthRadius::Mean,
                                                       Medium::AirDry1Atm,
                                                       Vector{rootCS, 0_T, 50_uT, 0_T});
  builder.setNuclearComposition(
      {{Code::Nitrogen, Code::Oxygen},
       {0.7847f, 1.f - 0.7847f}}); // values taken from AIRES manual, Ar removed for now

  builder.addExponentialLayer(1222.6562_g / (1_cm * 1_cm), 994186.38_cm, 4_km);
  builder.addExponentialLayer(1144.9069_g / (1_cm * 1_cm), 878153.55_cm, 10_km);
  builder.addExponentialLayer(1305.5948_g / (1_cm * 1_cm), 636143.04_cm, 40_km);
  builder.addExponentialLayer(540.1778_g / (1_cm * 1_cm), 772170.16_cm, 100_km);
  builder.addLinearLayer(1e9_cm, 112.8_km);
  builder.assemble(env);

into

  create_us_standard_atmosphere<setup::EnvironmentInterface, MyExtraEnv>(
      env, center, constants::EarthRadius::Mean, Medium::AirDry1Atm,
      MagneticFieldVector{rootCS, 0_T, 50_uT, 0_T});

This is still a WIP and all comments are welcome for improvements and alternative ways to approach this.

corsika/detail/media/LayeredSphericalAtmosphereBuilder.inl

@@ -40,8 +40,9 @@ namespace corsika {
   inline typename LayeredSphericalAtmosphereBuilder<TMediumInterface, TMediumModelExtra,
                                                     TModelArgs...>::volume_tree_node*
   LayeredSphericalAtmosphereBuilder<TMediumInterface, TMediumModelExtra, TModelArgs...>::
-      addExponentialLayer(GrammageType b, LengthType c, LengthType upperBoundary) {
+      addExponentialLayer(GrammageType const b, LengthType const scaleHeight, LengthType const upperBoundary) {
 
+    // outer radius
     auto const radius = planetRadius_ + upperBoundary;
     checkRadius(radius);
     previousRadius_ = radius;
@@ -49,14 +50,14 @@ namespace corsika {
     auto node = std::make_unique<VolumeTreeNode<TMediumInterface>>(
         std::make_unique<Sphere>(center_, radius));
 
-    auto const rho0 = b / c;
+    auto const rho0 = b / scaleHeight;
 
     if constexpr (detail::has_extra_models<TMediumModelExtra>::value) {
       // helper lambda in which the last 5 arguments to make_shared<...> are bound
       auto lastBound = [&](auto... argPack) {
         return std::make_shared<
             TMediumModelExtra<SlidingPlanarExponential<TMediumInterface>>>(
-            argPack..., center_, rho0, -c, *composition_, planetRadius_);
+            argPack..., center_, rho0, -scaleHeight, *composition_, planetRadius_);
       };
 
       // now unpack the additional arguments
@@ -64,7 +65,7 @@ namespace corsika {
       node->setModelProperties(std::move(model));
     } else {
       node->template setModelProperties<SlidingPlanarExponential<TMediumInterface>>(
-          center_, rho0, -c, *composition_, planetRadius_);
+          center_, rho0, -scaleHeight, *composition_, planetRadius_);
     }
 
     layers_.push(std::move(node));
@@ -75,7 +76,9 @@ namespace corsika {
             typename... TModelArgs>
   inline void LayeredSphericalAtmosphereBuilder<
       TMediumInterface, TMediumModelExtra,
-      TModelArgs...>::addLinearLayer(LengthType c, LengthType upperBoundary) {
+      TModelArgs...>::addLinearLayer(LengthType const scaleHeight,
+                                     LengthType const upperBoundary) {
+    // outer radius
     auto const radius = planetRadius_ + upperBoundary;
     checkRadius(radius);
     previousRadius_ = radius;
@@ -83,8 +86,8 @@ namespace corsika {
     auto node = std::make_unique<VolumeTreeNode<TMediumInterface>>(
         std::make_unique<Sphere>(center_, radius));
 
-    units::si::GrammageType constexpr b = 1_g / (1_cm * 1_cm);
-    auto const rho0 = b / c;
+    GrammageType constexpr b = 1_g / (1_cm * 1_cm);
+    auto const rho0 = b / scaleHeight;
 
     if constexpr (detail::has_extra_models<TMediumModelExtra>::value) {
       // helper lambda in which the last 2 arguments to make_shared<...> are bound