Python copy support

[remia]

This PR addresses #1434 adding copy / deepcopy (same implementation) for OCIO objects having C++ createEditableCopy() methods. It also fix the typo reported in #1540.

[zachlewis]

Thanks Remi, this is super useful. Seems to work as expected.

[hodoulp]

[Question] Only the deep copy exists in the public API of OpenColorIO i.e. createEditableCopy(), why also adding the Python copy()?

[remia]

Good question.

It seems that in our context, the difference between Python copy() and deepcopy() is not relevant and both may be accurately implemented with createEditableCopy(). At least that's how I understand it because I don't have a clear view on the object memory layout as generated by pybind11 and it may be unnecessary complexity to strictly follow Python definition of shallow copy.

Here is the pybind11 doc and a quote from the copy module doc:

The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

• A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
• A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

[hodoulp]

There is a major semantic difference between shallow and deep copies and, adding the two methods in Python introduces a discrepancy between the Python and C++ APIs (because the shallow copy is not supported in the C++ API). For example, a deep copy of a config instance means that the file rules are not shared between the two config instances (where a shallow copy would have share the file rule instance).

I think that adding the Python copy() (i.e. a shallow copy) introduces some confusion because it's in fact implemented by a deep copy.

[remia]

I can remove it, it will go back to produce a segfault as before if we are fine with it.

Edit: I removed the __copy__ method to avoid any confusion as suggested by @hodoulp.

[hodoulp]

LGTM. Only made some comments for code improvements.