Public subroutines that are read by the main stella routine.
Available physics options: These are standard gyrokinetic terms that
can be turned on/off with the following toggles.
Adiabatic options: This is used when nspec = 1. The non-kinetic
species (usually electrons) is set to have an adiabatic response.
This can be either the classic adiabatic option, or the modified
adiabatic option (i.e. modified Boltzmann electrons).