Reconsider design for custom methods

[dilpath]

At the moment, it's expected to be a class, that is then initialized with a function. Instead, we could expect some callable that accepts size/direction/point and returns the derivative.

fiddy/fiddy/derivative.py

Lines 177 to 183 in c015e4a

	method = custom_methods.get(
	method_id,
	methods.get(
	method_id,
	None,
	),
	)(function=function)