Remove the usage of non-portable std::map::at() from VariantMap
Remove the usage of non-portable std::map::at() from VariantMap

This removes the usage of the non-portable std::map::at() method
in the VariantMap::value() method. This fixes a compilation error
on windows.

Change-Id: Ifb5947af016f4f4e773a9a9721b979bea247b6bb

wadejong/avogadrolibs,OpenChemistry/avogadrolibs,wadejong/avogadrolibs,cjh1/mongochemweb-avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,wadejong/avogadrolibs,qust113/molcore,OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,cjh1/mongochemweb-avogadrolibs,wadejong/avogadrolibs,ghutchis/avogadrolibs,qust113/molcore,cjh1/mongochemweb-avogadrolibs,OpenChemistry/avogadrolibs,cjh1/mongochemweb-avogadrolibs