Merge pull request #39 from astrofrog/overlay-plotting

Improved analysis API to make it easier to use programmatically
More succinct metadata warning
Changed metadata={} default argument to metadata=None

Defaults are defined at class-definition, and should be immutable.
Otherwise, if the default is modified in place in any instance,
it propagates to all instances
Merge pull request #45 from ChrisBeaumont/cat_warn

More succinct catalog warnings
Allow custom merge test function to be passed
Fixed semantic logic
Make coords= and intensity= in merge function keyword arguments, and test for orphans with custom function.