Use of `get_gravity` `gt4py` function to select between field vs scalar gravity for whole atmosphere calculations

[fmalatino]

Currently gravity values are passed to stencils through a 3D field, which will either have all elements set to a constant gravity value, or will have adjusted gravity values if enable_wam has been passed as part of the DynamicalCoreConfig.

A method should be introduced to enable selection between using the field-adjusted structure when whole atmosphere calculations are selected and just scalar gravity when they are not, to reduce memory overhead.

Incoming whole atmosphere variable gravity changes in PR 140

[FlorianDeconinck]

If the gravity field is exclusively used within sencils, something like this would suffice

@gtscript.function
def get_gravity(gravity):
   if enable_wam:
      return gravity
   else:
      return CONST_GRAVITY 

def stencil(gravity: FloatField):
   ...
   a = get_gravity(gravity) * b

[fmalatino]

Should something similar also be added for rdg_var, as this has now also been prompted from a scalar to a field?