Fix the QE importer

[giovannipizzi]

When the CELL_PARAMETER line has the units using (at least one character in) upper case, it crashes (while it shouldn't).
The regex needs to be fixed.

Also, see how to deal cases with e.g. 'Mn1' and 'Mn2' properly.

[giovannipizzi]

This has been fixed by depending on qe_tools