ENH : add 'get_keys'
ENH : add 'get_keys'

Get a list of the data labels contained in the muggler

tacaswell/dataportal,ericdill/databroker,danielballan/dataportal,NSLS-II/datamuxer,ericdill/replay,NSLS-II/dataportal,ericdill/datamuxer,ericdill/replay,ericdill/datamuxer,ericdill/databroker,danielballan/datamuxer,NSLS-II/dataportal,danielballan/datamuxer,NSLS-II/replay,NSLS-II/replay,tacaswell/dataportal,danielballan/dataportal