ENH: Workflow splicer module

[mgxd]

This PR lays some of the groundwork for fMRIPrep extensions.

splice_workflow splices a constructed workflow, allowing developers to swap in alternative processing workflows. This works by finding all tagged Node/Workflow in a graph, and substituting the matching replacements workflow. This will require some work in the NiPreps to encapsulate each processing step, likely with a heavy dosing of buffernodes for input and output fields.

Additionally, the @tag decorator can be used on any init_...wf() factory to facilitate adding the _tag attribute.

[codecov]

Codecov Report

Attention: Patch coverage is 91.48936% with 12 lines in your changes missing coverage. Please review.

Project coverage is 71.45%. Comparing base (d2fda2a) to head (336f998).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
niworkflows/engine/splicer.py	90.27%	4 Missing and 3 partials ⚠️
niworkflows/engine/tests/test_splicer.py	92.64%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #938      +/-   ##
==========================================
+ Coverage   71.12%   71.45%   +0.33%     
==========================================
  Files          87       89       +2     
  Lines        8477     8618     +141     
  Branches     1057     1072      +15     
==========================================
+ Hits         6029     6158     +129     
- Misses       2220     2227       +7     
- Partials      228      233       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[oesteban]

Trying to wrap my head around this. I left a couple of comments.