Implementation of a templated magnetic field interface and UniformMagneticField type.

Following the examples in the ngc_demo project, I have implemented a UniformMagneticField mixin that can be added to the C8 environment hierarchy. This is tested in two unit tests in testEnvironment.cc here. This takes a fixed magnetic field vector in the constructor and returns that fixed magnetic field everywhere.

I'd appreciate your feedback @mreininghaus!

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