Adapt `cg_drag.f90` to accommodate drag forcing from various different approaches

[SimonClifford]

cg_drag computes the gravity wave drag, currently using the AD99 parameterisation scheme. We need to adapt it to accept:

• ML computation using directly coupled TensorFlow
• ML computation using directly coupled PyTorch
• ML computation using ForPy. The ForPy wrapper will call TensorFlow, PyTorch (or possibly something else).