Next: Improve error when calling a function that is not a field operator

[tehrengruber]

When a function, e.g. foo, called from a field operator, e.g. bar is accidentally not decorated as a field operator the error is not very user friendly:

gt4py.eve.exceptions.EveValueError: Symbols {SymbolRef('with_boundary_values')} not found.

Example:

def foo():
   ...

@field_operator
def bar():
   return foo()