Syncs mpiMcmc 'Leaving early' message for stage 2 with multiPopMcmc's
Additional performance improvements

Adds primary mass caching to deriveCombinedMags
Removes some calls to vector::size() in densities
Re-enables scatterCluster
Moves scatterCluster star culling into the loading stage

Removes the -1 sigma approach in favor of culling stars after loading. At the cost of generating s2n's for every star twice, only valid stars should now be run through the filter mutation code