Add some logic so HexAssemblies are hexagonal

[john-science]

Right now, the classes HexAssembly and CartesianAssembly are just empty slates:

armi/armi/reactor/assemblies.py

Lines 1229 to 1237 in 8b9873c

	class HexAssembly(Assembly):
	"""Placeholder, so users can explicitly define a hex-based assembly.
	.. impl:: Assembly of hex blocks.
	:id: I_ARMI_ASSEM_HEX
	:implements: R_ARMI_ASSEM_HEX
	"""
	pass

armi/armi/reactor/assemblies.py

Lines 1240 to 1241 in 8b9873c

	class CartesianAssembly(Assembly):
	pass

But they give people the assumption they can trust these assemblies have a shape, or the blocks inside them do. But obviously no.

I recommend that we override the superclass method Assembly.add() here to check the shape of the Blocks added to assemblies of these types. That will give users some trust, anyway, that instances of these classes are what you'd think at first glance.

[john-science]

@jeffbaylor @alexhjames Here is our ticket.