Remove inappropriate polymorphism of GetMetricInfo() from StdExpansion.
GetMetricInfo() only applies to Expansions, not to StdExpansions. However, it originally was put into StdExpansion.h and polymorphically implemented in Expansion. It doesn't make sense to get metric information about a StdExpansion, so we need to remove it from StdExpansion and place the non-virtual implementation in Expansion.h.
Specifically: replace the virtual version of v_GetMetricInfo() in Expansion.h with non-virtual GetMetricInfo(). Remove both the virtual (v_) and implementation of GetMetricInfo() from StdExpansion.