Refactor slz_json.py to remove metaclass and classvar abuse.

[laundmo]

Currently slz_json.py uses a metaclass which sets a classvar to track object ids and references.

This is not ideal, as it requires calls to reset to clear out the classvars between writing json files.

bonelab-mod-repo/modio_repo/slz_json.py

Lines 28 to 30 in 8357ef2

	def reset():
	for typ in refable_types.values():
	typ.elements = {}

the code should be refactored and re-thought completely, to start with a slz json base class which tracks the object ids for just that file, instead of globally.

Please comment on this issue if you want to work on it!