Make DataType's mutable

[evalott100]

We currently have frozen DataType's, with the callback to update the backend based on changed fields on the DataType taking a whole new DataType:

FastCS/src/fastcs/attributes.py

Lines 105 to 113 in bb6929a

	def update_datatype(self, datatype: DataType[T]) -> None:
	if not isinstance(self._datatype, type(datatype)):
	raise ValueError(
	f"Attribute datatype must be of type {type(self._datatype)}"
	)
	self._datatype = datatype
	for callback in self._update_datatype_callbacks:
	callback(datatype)

We should instead make the DataType mutable and have the above method just use self._datatype.