implement parsing methods

[kjappelbaum]

for testing inverse design models, we will ultimately need to parse the different text representations. That is, we would need methods that can read the various text formats you implemented and give us, as much as possible, a pymatgen object.

I think we can certainly implement a from_tuple classmethod.

We need to think of the best way to deal with the other things (/cc @n0w0f)