Move equation-specific limiter functions from Limiters.cpp to DGMultiMat, DGCompFlow or DGTransport

[adityakpandare]

Equation-specific limiter functions should reside in the DG-equation classes rather than being free functions in Limiter.cpp See comment from @jbakosi in #496:
"Based on the function name, this should probably be in dg::MultiMat. Will this be called (i.e., should this be accessible) from anywhere else?"