Add slice-time correction to preprocessing

[mwaskom]

This should be added while remaining optional. A few thoughts:

• Probably easier to do in Python with spline interpolation than calling out to FSL.
• Need to determine the best way to specify slice times
• Should it be it's own node or should it be wrapped into the FinalizeTimeSeries interface to minimize loading/saving of the time series?

[mwaskom]

See http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0182939

What probably make sense is to do slice time correction in the first step of the preprocesing workflow, then write out the individual volumes of the slice-time corrected data (for later resampling) and the full time series of the uncorrected data (used to estimate motion but not later resampled).