Implement a replacement for Fortran PRESENT(X)

[FlorianDeconinck]

Fortran has the concept of a PRESENT(X) to test if argument X was passed to a function. Functionally it's the Optional[type] or the None of Python, which cannot be processed by GT4Py.

The usage would be:

def my_stencil(
  a_field: FloatField
  b_optional_value: Optional[float]
):
  if b_optional_value is None:
    a_field = 10
  else:
    a_field = 20

Code generation in C could be a duplicate of the flag into a do_X and value_X with resolution at compile time?

The workaround for this right now is to use an __external__ to be able to flip some code BUT it only works on compile-time defined flags.

Parent: #36

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• Optional[float] is a supported feature, with potential restrictions documented and feedback cleanly