Python `is` construct is forbidden and should error out at `gtscript` level

[FlorianDeconinck]

The following code should error at gtscript level:

def (a_bool: bool, x: FloatField):
     with computation(PARALLEL), interval(...):
         if a_bool is True:
             x = 42

Parent: GEOS-ESM/SMT-Nebulae#36

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• Fail with a clean feedback for the user