Improve documentation on MatchOnce keyword (#538)
Improve documentation on MatchOnce keyword (#538)

Add an extended description of MatchOnce with a
worked example to the docstring.

pysb/pysb,johnbachman/pysb,LoLab-VU/pysb,jmuhlich/pysb,alubbock/pysb,johnbachman/pysb,LoLab-VU/pysb