Error when calculating fqt using isotopes

Currently, when using a universe containing isotopes, there is an error which occurs in FQt._set_weights, as well as FQtCoherent._set_weights and FQtIncoherent._set_weights (fqt.py,fqt_coh.py, fqt_incoh.py, respectively).

This error is due to trying access an invalid isotope from the periodictable database. This is purely an issue with the form of the string rather than anything deeper. For example the isotope will be written with string '36-Ar' but periodictable expects .....element('Ar)[36] to access any information.