Apply Cp(T) to generic physics

[jvatant]

Following what has been done for Venus physics and set in dynamics for cp(T)=cpo(T/To)^nu we can apply this for generic physics based on already-included cpdet_mod.F90

This is for instance a major requirement for ice giants.

Things to check :

• coherence between physics and dynamics (force_ccp_mu?)
• calculation of cp(T) for one gaz only and take into account the other gases with constant cp (not able for now in dynamics where only the total cp varies ?)

Enhancements to provide :

• For existing cp(T) include To and nu as input in run.def to be no more Venus-hard-coded
• Enable cp to vary as polynomial sum cp(T)=sum(a*Tⁿ), which is commonly encoutered, and derive the t2tpot and tpot2t accordingly