Add support for different coordinate systems to more function plugins

[gassmoeller]

I was made aware that some of ASPECT's function plugins support choosing a different coordinate system (like defining the function in dependence of spherical coordinates), while others do not. This is understandable, because this functionality was added piecewise, whenever it was needed, but it is also confusing to new users. It should be relatively easy to copy-paste the existing code to all function plugins that currently do not support different coordinate systems.

I looked through the plugins and these are plugins that currently do not support different coordinate systems:

• adiabatic_conditions/function.cc (but this one is a 1D function in dependence of depth, so it doesnt need it).
• geometry_model/initial_topography_model/function.cc
• gravity_model/function.cc (unlikely to be needed, but who knows)
• heating_model/function.cc (this was the one reported to me, so it would be needed)
• mesh_deformation/function.cc
• particle/property/function.cc
• prescribed_stokes_solution/function.cc

Each one could be a nice starter project for a first time contributor.