[FEATURE] Parameter for Galaxy Simulation Inflow Metallicity

[clairekope]

(Modified from a comment in PR #216) A parameter which controls the metallicity of the inflowing gas in the GalaxySimulation problem should be created and connected. This will require changes to GalaxySimulationInitialize.C, Grid_GalaxySimulationInitialize.C, and ExternalBoundary_SetGalaxySimulationBoundary. In particular, a new global parameter in the GalaxySimulationRPSWind family would need to be created, set and used in those routines. Currently the metallicity inflow is hard-wired to 0.1 solar as the least bad thing.