#548 Add the ACS correlated noise file to the share directory

This is a pretty easy PR.  For one of Josh's issues, we created a new share directory where non-code files can be placed that we want to be able to access from within GalSim.  Josh used it for a Vega SED to enable Vega magnitude calculations.  It seems appropriate to also have the noise file for `getCOSMOSNoise()` installed there, so users don't have to point to a specific file in the repo.

That's what this PR implements.  Now you can use `getCOSMOSNoise` with no `file_name` parameter, and it will use the file in the share directory.

I also caught a slight error in the CorrelatedNoise class that it wasn't properly allowing the obsolete `dx` syntax as we had claimed it was, so I fixed that.