SDC: phase1/2, phasediff and fieldmap should share a common final stage

[oesteban]

Simplify this:

https://github.com/poldracklab/fmriprep/blob/9c57dded04c7c0f26768264afaaee1d4aa158b93/fmriprep/workflows/fieldmap/base.py#L202-L241

At this time, init_fmap_wf and init_phdiff_wf are very similar. The idea would be to implement:

• Magnitude registration; and
• regularization and displacements field calculation

only once on init_fmap_wf from a fieldmap in Hz. Phase1/2 and phasediff would then focus on phase unwrapping and conversion to Hz.

[mattcieslak]

Both are there in #30

[oesteban]

Hey @mattcieslak, I think I'm going to split this from #30, which is too fat of a PR.