minor refactoring

3 jobs for master in 37 minutes and 47 seconds
Name Stage Failure
test Test
  A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
df['value'] = transformations[transformation](df['value'])

-- Docs: http://doc.pytest.org/en/latest/warnings.html
======== 1 failed, 255 passed, 10 skipped, 10 warnings in 99.88 seconds ========
ERROR: Job failed: exit code 1