Add support for different Earth radii approximations into LayeredSphericalAtmosphereBuilder

LayerSphericalAtmosphereBuilder currently uses a fixed earth radius (the mean Earth radius of 6371 km) implemented as a static constexpr - it also supports a seaLevel member variable that is used as an Earth radius everywhere in the code (and defaults to the fixed Earth radius). There are several experiments that are sensitive to the Earth radius used to approximate the Earth's spheroid (like ANITA and PUEO) and there are other applications where we could reuse the layered atmosphere builder at other radii.

This MR removes the static constexpr earth radius and makes it an argument to the constructor (replacing the equivalent seaLevel that was used as an Earth radius). It also creates a nested namespace containing constexpr values for the various Earth radius models that are useful (polar radius, mean radius, etc.). Access to the constexpr value of earthRadius is replaced with a method getEarthRadius().

This is ready for review. (Sorry if you were spammed with notifications - I had to rebase onto the enormous MR that happened recently).

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