WIP: Switch from nistats to nilearn

[rob-luke]

Switch from Nistats to Nilearn, as nistats is being depreciated. As discussed in #61.

[codecov-io]

Codecov Report

Merging #63 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   24.13%   24.13%           
=======================================
  Files           7        7           
  Lines          87       87           
=======================================
  Hits           21       21           
  Misses         66       66           

Impacted Files	Coverage Δ
...e_nirs/experimental_design/_experimental_design.py	11.53% <0.00%> (ø)

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