Disallow computations on field operator arguments in program

[tehrengruber]

This currently fails

@program
def program_domain(a: Field[[IDim, JDim], float64], my_scalar: float64):
    fieldop_domain(my_scalar + 1., a, out=a, domain={IDim: (1, 9), JDim: (4, 6)})

program_domain(a_IJ_float, 1.0, offset_provider={})

with the following error

E           TypeError: 'StencilClosure.inputs' must be <class 'gt4py.next.iterator.ir.SymRef'> (got 'my_scalar + 1.0' which is a <class 'gt4py.next.iterator.ir.FunCall'>).

../../src/gt4py/eve/type_validation.py:363: TypeError

We should either allow this (probably tricky) or give an error earlier in the toolchain.