Weight expressions aren't checked syntactially

[BenWeber42]

This code will crash dusk:

@stencil
def invalid_weights_expression(a: Field[Edge], b: Field[Cell]) -> None:
  with domain.upwards:
    a = sum_over(Edge > Cell, b, weights=[1, -1]/2)

The check is missing here:

dusk/dusk/grammar.py

Line 897 in 8192508

weights = [self.expression(weight) for weight in kwargs["weights"].elts]

(kwargs["weights"] needs to be checked syntactially)