Implement multi-index (bandpass-like) graph split

[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 #18 (be05e07) into master (cf27a74) will decrease coverage by 0.07%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   76.55%   76.48%   -0.08%     
==========================================
  Files          13       13              
  Lines        1062     1080      +18     
==========================================
+ Hits          813      826      +13     
- Misses        249      254       +5     

Impacted Files	Coverage Δ
nigsp/operations/timeseries.py	79.34% <68.42%> (-2.94%)	⬇️
nigsp/utils.py	95.34% <100.00%> (+0.61%)	⬆️