Add 'cumulative percent' contribution analysis mode

[marc-vdm]

Adds 'cumulative percent' as a contribution analysis type in AB.

current types:

• Relative (refactored to Percent): shows contributors of at least n%
• Top # (refactored to Number): shows highest n contributors

New type would show a cumulative percentage, all (sorted) contributions that together reach at least n%.
e.g. in the process contributions example a cumulative percent set to 60% would only show steel production, with 80% it would show both steel production and electricity production.

This is relevant to add as e.g. the PEF regulations require the cumulative 80% contributors to be reported (chapter 6.3):

The most relevant processes are those that collectively contribute to at least 80% to any of the most relevant impact categories identified

Issues:

• resolve contribution analysis, add another cutoff method #1269
• resolve Add cumulative percent mode to contribution analysis #1431

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update the documentation
  • For in-code documentation, please follow the numpy style guide.
  • For user documentation, please update the wiki in
    ./activity_browser/docs/wiki
• Update tests.
• Link this PR to related issues by using closing keywords.

If you have write access (otherwise a maintainer will do this for you):

• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Add a milestone to the PR (and related issues, if any) for the intended release.
• Request a review from another developer.

[coveralls]

coverage: 52.972% (-0.1%) from 53.118%
when pulling 43db9dc on marc-vdm:cum_percent_ca
into eeb1685 on LCA-ActivityBrowser:main.