WIP: Parallelize tracking

[arokem]

Along the way, this also fixes a couple of bugs in mask handling in csd/dti, and allows passing an already-initialized direction getter object into the tracking, in case that's a thing you want to do (I use it to inject a bootstrap direction getter into the tracking, without giving it the raw data).

[pep8speaks]

Hello @arokem! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file AFQ/tractography.py:

Line 36:1: E402 module level import not at top of file

Comment last updated at 2021-04-19 18:21:46 UTC

[arokem]

One challenge with the parallelization done this way is that I can't seem to be able to use other backends than "threading" because the direction getter and tracker objects that get sent into the parallelized function cannot be pickled by python. I still need to do some more experiments with this to see whether this even provides any performance gains.

[arokem]

I should try out pathos, which uses dill to serialize Python objects. Might get past the issues I am seeing here now.

[arokem]

I am trying to make the Cython objects in DIPY pickle-able, based on https://ask.sagemath.org/question/8376/pickling-extension-classes/, but still doesn't work yet.