Incorrect type of key in `StartStopEntry` for representing a key that may not be present

[yousefmoazzam]

Same as #608, but for the start_offset and stop_offset keys in StartStopEntry:

httomo/httomo/transform_loader_params.py

Lines 34 to 42 in 584e15c

	class StartStopEntry(TypedDict):
	"""
	Configuration for a single dimension's previewing in terms of start/stop values.
	"""
	start: Union[int, str]
	start_offset: Optional[int]
	stop: Union[int, str]
	stop_offset: Optional[int]

Optional[int] should be replaced with NotRequired[Optional[int]] to correctly reflect the state of the dict that can be received from the start/stop config for a dimension in a preview parameter value.