attempt to speed up the generation of new columns

moved some of the computationally expansive blocks around
- matching on differenttimes in merge_lc is only required if we actually
  make an lc of more than 1 band
- copy lightcurve data in calc os only required is data_preprocessor is passed
  and that is a rare case.
fix calc docstring
Merge branch 'master' of github.com:YSOVAR/YSOVAR