Extracting KZAs directly from ENDF files, independently from filename

[eitan-weinstein]

This PR follows conversations from #271 and #279 to identify nuclides directly from the data contained within their ENDF files, rather than relying on them being named correctly. As it stood, relying on files to be named correctly was a rather fragile system, so this PR should provide a more stable basis by which to establish nuclide identity in the form of KZA, and then subsequently extract all other identifying characteristics (chemical symbol, mass number, isomeric state) from the KZA itself.

[gonuke]

I think this looks pretty good with just a few suggestions.

[gonuke]

Why not convert to strings after doing integer math?

Suggested change

	kza = str(kza)
	A = kza[-4:-1]
	Z = kza[:kza.find(A)].zfill(2)
	element = list(elements.keys())[int(Z) - 1]
	M = int(kza[-1])
	M = kza % 10
	ZA = kza // 10
	A = kza % 1000
	Z = kza // 1000
	element = list(elements.keys())[Z - 1]

[gonuke]

Related - could we just use the integer value of Z everywhere that we currently use element?

[eitan-weinstein]

Good point on the integer math. Much more intuitive. Certainly, we could use Z everywhere instead of element, although, I think I like using element, especially for printouts and warnings because it is easier for the user to see something like "Fe-56" rather than "26-56" as an identifier and then having to map the Z to the chemical symbol themselves.

[gonuke]

LGTM - thanks @eitan-weinstein