[ENH] Generate svgs only

[oesteban]

Make it easier for FMRIPREP to strip svgs from the reports.
HTML files will be used for reportlets (like the summary) that
can be completelly inlined. Otherwise, expect standalone svgs.

[chrisgorgo]

Could you open WIP PRs for MRIQC and FMRIPREP using this branch so we would see how this is working?

[oesteban]

There already (nipreps/fmriprep#867):

• https://435-53608443-gh.circle-artifacts.com/0/home/ubuntu/ds054/out/fmriprep/sub-100185.html
• https://435-53608443-gh.circle-artifacts.com/1/home/ubuntu/ds005/out/fmriprep/sub-01.html

EDIT: MRIQC only uses the registration report, which was already in svg.

[chrisgorgo]

So does this help with large reports that previously were slow to load or crashed browsers?

[oesteban]

The load time is definitely shorter. I can't claim though that this solves the problem until I see a huge dataset like MyConnectome using this. But that would be an issue of fmriprep, not in niworkflows.

This PR modifies niworkflows:

• to make it more consistent (ends the dichotomy of registration vs. segmentation reports or svg vs. html respectively),
• to reduce the codebase and simplify functionality (we can stop caring about html validity, etc)
• to simplify the tests of report-capable segmentations by leveraging py.tests, as we previously did with tests if registration interfaces.

[chrisgorgo]

Yes it does seem like a major refactor. A bit too much for me to dig through right now, but if it works and does not break MRIQC and FMRIPREP you should be good to go.