[WIP] Remove the use of meta classes in the ProcessBlock class hierarchy

[jsiirola]

Fixes

Summary/Motivation:

The use of a metaclass for defining process blocks was both confusing and caused unintentional errors (conflicting metaclass definitions that faught with the RenamedClass backwards compatibility wrappers. This PR reworks the magic for declaring process block classes to:

• remove the use of metaclasses
• declare unique instances of the Scalar and Indexed block classes
  • ensure the Scalar and Indexed classes are added to the module scope (along with the generic derived Block object)
• improve general documentation and code clarity

Changes proposed in this PR:

• (see above)

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[lbianchi-lbl]

Suggested change

	return super().__new__(cls._scalar_process_block, *args, **kwargs)
	# pylint: disable-next=no-member
	return super().__new__(cls._scalar_process_block, *args, **kwargs)

[lbianchi-lbl]

Suggested change

	return super().__new__(cls._indexed_process_block, *args, **kwargs)
	# pylint: disable-next=no-member
	return super().__new__(cls._indexed_process_block, *args, **kwargs)

[andrewlee94]

Closing this as low priority for now.