added comments only
replicate Mappint into NMMapping as specifically the type for NONMEM dataset mapping type

removed CovariateMapping from the general Mapping type, now move to the
NMMapping type
NONMEMvariableType to NONMEMsymbolType

CovariateMappingType to NMCovariateMappingType
CovariateMapping to NMCovariateMapping
NMVariableMapping to NMsymbolMapping