Make VISxIR bands dynamic (defined at run time and not compile time)

[emillour]

Currently the number of bands in the visible and IR are set at compile time by the "-b ...x..." option of makelmdz_fcm
But it would be more convenient to have these set dynamically via callphys.def and not have to recompile the model when changing band numbers.

Moreover there is no trace of number of bands the model was compiled with, so an in-between (before going full dynamically allocated) would be to add "_bVISxIR_" in the name of the gcm.

[emillour]

Adding "_bIRxVIS" to the name of the executable done in r2967