ENH: Add high-level predictor fetching utilities

[adelavega]

Adds a fetch_utils module, with the function fetch_predictors & fetch_images

This helper simplifies using api.predictor_events to fetch predictors.

After fetching predictors using predictor_events, it does the following:

• Grab duration, and TR from API for each unique run (from /api/tasks), and compute the probably n_vols for each. Note: ideally this would be grabbed directly from the volumes, rather than computed, but I didn't record this in the API. I could do this in the future, but need to add this to the API.
• Creates a BIDSVariableCollection (pybids) from these Predictors, to facilitate resampling to TR (without reimplementing this logic).
• Can return BIDSVariableCollection if the user wants to apply Transformations (e.g. scale), or can return a df (resampled or not).

The main problem I see here is the pybids dependency, but I think this will help users grab TR resampled timeseries.

fetch_images simplifies the process of fetching images from Neuroscout using datalad by:

• Given the name of a dataset and a path:
• Installing preproc dataset
• Fetching files given a set of entities
• Uses pyBIDS to associate paths w/ entities (to match to events easily)

[adelavega]

@jsmentch would you find this useful?

[jsmentch]

Yes definitely - I was previously doing resampling to TR manually so the resampling alone would be convenient in the future

[adelavega]

Cool. Also check out this PR: rbroc/neuroscout-encoding-models#2

This is the workflow roberta was working on w/ himalaya. I don't think I'll implement a full workflow for Neuroscout yet, but with this helper function it should be easy for people to use the data as they wish.

[codecov-commenter]

Codecov Report

Merging #112 (2d08071) into master (56bd1a5) will decrease coverage by 11.13%.
The diff coverage is 0.91%.

@@             Coverage Diff             @@
##           master     #112       +/-   ##
===========================================
- Coverage   87.10%   75.97%   -11.14%     
===========================================
  Files          17       18        +1     
  Lines         737      845      +108     
===========================================
  Hits          642      642               
- Misses         95      203      +108     

Impacted Files	Coverage Δ
pyns/endpoints/base.py	69.90% <ø> (ø)
pyns/fetch_utils.py	0.00% <0.00%> (ø)
pyns/__init__.py	100.00% <100.00%> (ø)

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