Use NIST ionization potentials, not phfit.dat

Previously, Cloudy used various sources for ionization potentials: the ionization_potentials.dat file that uses NIST values; the (valence) shell ionization energies in phfit.dat (by Dima Verner); and for the H and He iso-sequences, values embedded in the data files (.nrg.dat, in data/stout/).

In one instance this has led to a bug with the set continuum resolution command.

This branch eliminates use of other sources of IP data in favor of the NIST values.