Identical levels

[mbradle]

When lvlspy updates data from XML, it will overwrite an existing level that has the same energy and multiplicity as a new level. When the new level has the same energy but different multiplicity, the new level will be added without removing the old level. This will cause an issue with transitions.

Such a case only arises because the input is correct, but it would be useful to check or handle this somehow. Possibilities are in the schema, in the equality for the levels, or outside the API.