[python] Inconsistent use of derivative vs derivatives

[jeffersoncasimir]

There are multiple locations where derivatives is used in place of derivative, which is the correct usage according to the BIDS spec.

This was discovered while testing with RB data, where the physiological_output_type is set to be derivatives. It breaks in one location when running without the -t set to either derivative or derivatives, and only with RB data since the regular Loris DB's physiological_output_type field doesn't include the s when initially added.

It would break more seriously if the -t parameter is set to either variation.

The fix on the Loris-MRI side is to change derivatives to derivative in all string comparisons in all files under python/.

Found when testing #1078.