Add outflow random walk normalisation (and specify former random walk as inflow random walk)

[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 #54 (9efbf92) into master (fdaa106) will decrease coverage by 0.15%.
The diff coverage is 50.00%.

❗ Current head 9efbf92 differs from pull request most recent head 06498bc. Consider uploading reports for the commit 06498bc to get more accurate results

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   91.33%   91.19%   -0.15%     
==========================================
  Files          14       14              
  Lines        1212     1215       +3     
==========================================
+ Hits         1107     1108       +1     
- Misses        105      107       +2     

Impacted Files	Coverage Δ
nigsp/operations/laplacian.py	93.67% <50.00%> (-2.39%)	⬇️