Added AVO_NULLPTR, minor dox update
Added AVO_NULLPTR, minor dox update

Change-Id: I1e3386fb236eded4b5101cab46c1cc8217ea97b0

ghutchis/avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs,wadejong/avogadrolibs