ENH: return stc when applying stc.transform_data
STY
ENH: add transform method to SourceEstimate class
WIP: add transform method to SourceEstimate class
WIP: add transform method to SourceEstimate class
WIP: add transform method to SourceEstimate class
WIP: test SourceEstimate transform method
WIP: test SourceEstimate transform method
WIP: test SourceEstimate transform method
Merge pull request #842 from adykstra/return_stc_data_transform

ENH:  add transform method to SourceEstimate class