[ENH] Effective number of assets

[CarloNicolini]

Reference Issues/PRs

Issue #13 Implement effective number of assets

What does this implement/fix? Explain your changes.

The measure of portfolio concentration called effective number of assets. However I need to discuss how to embed this calculation in the portfolio.summary as it requires the asset weights, not the portfolio returns.

Does your contribution introduce a new dependency? If yes, which oneNo

What should a reviewer concentrate their feedback on?

The reviewer should indicate how to include this metric in the set of already existing metrics, given that it requires a different argument from simple portfolio returns.

Did you add any tests for the change?

No

For all contributions

• [x] The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• [ ] I've added myself to the list of contributors How to: add yourself to the all-contributors file in the skfolio root directory (not the CONTRIBUTORS.md).
• [ ] Optionally, I've added myself and possibly others to the CODEOWNERS file - do this if you want to become the owner or maintainer of an estimator you added.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (60cd559) 99.14% compared to head (ddd5c26) 99.14%. Report is 13 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #17 +/- ## ======================================= Coverage 99.14% 99.14% ======================================= Files 35 35 Lines 2584 2584 ======================================= Hits 2562 2562 Misses 22 22 ```

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