Avoid repeated code

[MarcoArtiano]

As the number of set of equations we are able to solve increases, some code is repeated and could be easily avoided. As an example for the baroclinic test case the initial conditions and auxiliary functions are repeated, as they are written in terms of primitive variables.

See for example
cartesian_to_sphere in Euler Energy

cartesian_to_sphere in Potential Temperature

The same applies to source terms and initial conditions, which partially relates this issue also to [#120].

[benegee]

Yes, right!

This should probably go to some auxiliary file. Along with methods to also transform velocity vectors, etc..