Do not output files when comparing skymodels

[gmloose]

The current SkyModel.compare() method always outputs several files. This may not be desirable.

One way to make this behavior optional is to set the outDir=None instead of outDir='.' in compare(). Then, plots should only be made if outDir is not None. Similarly, the stats.txt file should only be created if outDir is not None.

By changing the behavior of compare(), we do not break code that uses the run() method, because that sets 'outDir to . by default. Still, this is a change in behavior, so should be discussed before being accepted.

[darafferty]

That sounds like a good way to do it to me. I'm not aware of any existing pipelines, etc. that would break due to the proposed changes.