Add possibility (1) to specify which diagonal to use for matrices symmetric normalisation and (2) to avoid "0" elements in the diagonal

[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.

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

[codecov]

Codecov Report

Merging #29 (db5c276) into master (d990f48) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   90.50%   90.61%   +0.11%     
==========================================
  Files          14       14              
  Lines        1053     1066      +13     
==========================================
+ Hits          953      966      +13     
  Misses        100      100              

Impacted Files	Coverage Δ
nigsp/operations/laplacian.py	100.00% <100.00%> (ø)