Allow import of X number of electrons from Java database.

Translates to the union of electronic states containing 0, 1, 2S, or 2T.
We expect X to be used only in describing these electronic states,
since Java does not have any nitrogen containing groups.

When importing from Java, the X gets translated to {0,1,2S,2T}.
Conversely I believe that typing X in the Py-style database should also work
to do the same thing now.