Gafferpy tidy up operation parameter handling

[t92549]

Currently in the Gafferpy codebase there is a non-pythonic pattern of processing operation parameters by doing something like this:

gaffer-tools/python-shell/src/gafferpy/gaffer_operations.py

Lines 2381 to 2388 in bbf0634

	if conditional is not None:
	if not isinstance(conditional, Conditional):
	self.conditional = JsonConverter.from_json(conditional,
	Conditional)
	else:
	self.conditional = conditional
	else:
	self.conditional = None

Not only is this over-complicated, but it causes bugs when one of those cases is missed (#870).
Fishbowl does not have this issue due to the nature of it having no error checking and presuming everything is json.

@sw96411 suggested an improvement on #890 (comment) which is more pythonic, easier to read and less bug prone.

[t92549]

Also worth noting that while changing to this approach, it should be easier to avoid bugs like in #851.