Change how ambient medium is treated - now need to explicitly set which components to subtract from ambient medium - allows multiple ambient mediums (e.g. for different dust properties) to now be specified.
Updated AnalyticalYSOModel to support updated ambient density class
Added tests for ambient density and fixed bug in PowerLawEnvelope (should not renormalize if rho_0 is explicitly set)
Added docs for ambient medium
Merge pull request #71 from astrofrog/multiple-ambient-medium

Allow multiple ambient density mediums