MAINT: Purge Cython and depend on SDCflows

[effigies]

Changes proposed in this pull request

We've moved all the fieldmap tools into sdcflows, so we don't need the Cython extensions in fMRIPrep. This removes maths.pyx and all code that depends on it, and updates fmriprep.interfaces.fmap to import its contents from sdcflows.interfaces.fmap.

Cython and numpy have been removed from the build requirements, and the wheel built on CircleCI will be pure Python, so we can drop manylinux.

Documentation that should be reviewed

[effigies]

@oesteban I don't know what's up with the ds005 and ds210 errors. Looks like some other nipype regression.

[oesteban]

I'm pretty positive those errors will go away if we clean the cache. We should use this as an opportunity to debug nipy/nipype#3014 (this is an instance of this issue). Can you think of a way to retrieve Circle's cache and take it to a controlled environment?

[effigies]

Just create a job that loads the cache and saves it as an artifact? Could tarball it for easy download.

[oesteban]

Just create a job that loads the cache and saves it as an artifact? Could tarball it for easy download.

I think Circle tells you the identifier of the cache it found and retrieved. We may want to have a build that finds a cache identifier on the commit message and tarballs the cache. Nothing otherwise. Could be helpful in the future. WDYT?

[oesteban]

I've removed one file that wasn't necessary anymore and updated the sdcflows pin.

[oesteban]

Since I rebased to upstream/master, these are the links to the broken builds:

https://circleci.com/gh/poldracklab/fmriprep/9753?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
https://circleci.com/gh/poldracklab/fmriprep/9756?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link