cartesian: improve errors when accessing fields with float literals / expressions

[romanc]

Description

We got a false positive user report, which tracked down to the usage of float literals/expression in field offsets. Field offsets are expected to be of integer type. All eve nodes support datamodel validation. We could thus implement consistent checks for all field offsets (CartesianOffset, VariableKOffset) to either be an integer literal or evaluate to an integer expression.

Currently, we have such a datamodel validator for the "K" offset of VariableKOffset

gt4py/src/gt4py/cartesian/gtc/common.py

Lines 322 to 345 in cef9dee

	class CartesianOffset(eve.Node):
	i: int
	j: int
	k: int
	@classmethod
	def zero(cls) -> CartesianOffset:
	return cls(i=0, j=0, k=0)
	def to_dict(self) -> Dict[str, int]:
	return {"i": self.i, "j": self.j, "k": self.k}
	class VariableKOffset(eve.GenericNode, Generic[ExprT]):
	k: ExprT
	def to_dict(self) -> Dict[str, Optional[int]]:
	return {"i": 0, "j": 0, "k": None}
	@datamodels.validator("k")
	def offset_expr_is_int(self, attribute: datamodels.Attribute, value: Any) -> None:
	value = typing.cast(Expr, value)
	if value.dtype is not DataType.AUTO and not value.dtype.isinteger():
	raise ValueError("Variable vertical index must be an integer expression")

This PR suggests to use a similar validator for the members of CartesianOffset.