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.
Renamed merge_test_function to is_independent
Update API for ``is_independent`` function, and added docstring
Added simple test for ``is_independent``
Use benchmark examples to further test is_independent
Added section to docs about using ``is_independent``
Merge pull request #10 from astrofrog/custom-merge-function

Example custom merge test function