Added basic derived quantities for subfind hdf

For example, new metallicity quantities, that can now be used in
creating Fe/H plots within existing pynbody analysis routines
added abundance array translations to default_config.ini
Added metallicity shortcut names and changed derived quantites to these

Still issue regarding ionised hydrogen fraction will add CLOUDY table
Added metal shortcuts and ion fractions
Added Gadget3 Solar abundances
Merge pull request #2 from rokroskar/subfind_hdf_duff

Subfind hdf duff