Convert ANTs commands to use antspy

[mikarubi]

since we got rid of spark as a dependency, it makes sense to do same for ants, since it has a simple wrapper in python. https://pypi.org/project/antspyx/

it should be straightforward to convert the corresponding ants calls into python. i can give it a shot, and the advantage would be a package that can be fully installed via pip without need to install other software.

Originally posted by @mikarubi in #26 (comment)

[luiztauffer]

that would be great! Users wouldn't need to manually install and set PATH to ANTs

[luiztauffer]

As of now, the tests are mostly checking if the code runs successfully and asserting that the expected files are created. I suggest we start including assertions on the content of these files, to guarantee that such a modification will work as expected

[mikarubi]

Agree. I believe that the pipeline is largely deterministic, except for:

step 2: ants_registration (https://github.com/ANTsX/ANTs/wiki/antsRegistration-reproducibility-issues)).
step 3: GaussianMixture
step 4: AgglomerativeClustering
step 4: linalg.lstsq

For the purposes of such a test, we could run omit set registration='none' and set random number seeds to ensure reproducibility in other steps.