Add style check test (flake8)

[smoia]

Closes #

Proposed Changes

Change Type

• bugfix (+0.0.1)
• minor (+0.1.0)
• major (+1.0.0)
• refactoring (no version update)
• test (no version update)
• infrastructure (no version update)
• documentation (no version update)
• other

Checklist before review

• I added everything I wanted to add to this PR.
• [Code or tests only] I wrote/updated the necessary docstrings.
• [Code or tests only] I ran and passed tests locally.
• [Documentation only] I built the docs locally.
• My contribution is harmonious with the rest of the code: I'm not introducing repetitions.
• My code respects the adopted style, especially linting conventions.
• The title of this PR is explanatory on its own, enough to be understood as part of a changelog.
• I added or indicated the right labels.

• I added information regarding the timeline of completion for this PR.

• Please, comment on my PR while it's a draft and give me feedback on the development!

[codecov]

Codecov Report

Merging #25 (b996146) into master (f2d5742) will not change coverage.
The diff coverage is 87.50%.

❗ Current head b996146 differs from pull request most recent head 451713e. Consider uploading reports for the commit 451713e to get more accurate results

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   90.40%   90.40%           
=======================================
  Files          13       13           
  Lines        1042     1042           
=======================================
  Hits          942      942           
  Misses        100      100           

Impacted Files	Coverage Δ
nigsp/objects.py	100.00% <ø> (ø)
nigsp/operations/metrics.py	100.00% <ø> (ø)
nigsp/operations/timeseries.py	92.47% <66.66%> (ø)
nigsp/io.py	91.12% <100.00%> (ø)
nigsp/operations/laplacian.py	100.00% <100.00%> (ø)
nigsp/operations/nifti.py	96.55% <100.00%> (ø)
nigsp/operations/surrogates.py	89.91% <100.00%> (ø)
nigsp/utils.py	95.12% <100.00%> (ø)