Added AVOGADRO_PLUGIN_DIR as an env variable
Added AVOGADRO_PLUGIN_DIR as an env variable

Check AVOGADRO_PLUGIN_DIR, and override any other base directory if it
is set. This makes testing more reliable, and could be useful where the
relative pathing is not desirable.

Change-Id: I536c078fe7a0429be2e2cec4586c69c77c729b13

ghutchis/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs