BUG: Fix regression causing failure to read incomplete vasprun.xml

Fix a regression related to parsing incomplete vasprun.xml, e.g. from a still-running calculation. Prior changes allow read_vasp_xml to include <calculation> blocks missing their close-tags, but did not check to see whether these blocks contain the necessary energies/positions/forces. Now read_vasp_xml will throw away <calculation> blocks that lack a final energy, which occurs at the end of the block.