Add new function fetch_megafon

[ezhdi]

---

name: "Add new function fetch_megafon"
about: Add Megafon dataset in sklift.datasets described in issue #99

📑 Description of the Change

Add new function fetch_megafon in sklift.datasets that loads and return train part of the Megafon dataset

Verification Process

Release Notes

Additional info

[maks-sh]

Hey!

Thank you 🙌 Amazing work 👍

There are a couple of small comments:

1. Since we do not have the ability to select a target, it makes no sense to create the `target_col ' argument. The fetch_x5 function is made similarly.
2. Unfortunately, now the documentation pages are not created automatically. Therefore, when adding new functions, you need to manually add a page to the desired directory. In your case, this directory is scikit-uplift/docs/api/datasets/. Please create a page along the way with content similar to fetch_x5 function.
3. It would be great to add the Key figures section to descr/megafon.rst by analogy with the criteo dataset (Format, Size, Rows, etc.).

I apologize that these items were not described in the issue.

[ezhdi]

Hi! fix all comments and bug when import all datasets too

[ezhdi]

Add example of usages for fetch_megafon to the docstrings

[maks-sh]

Thank you for your contribution!

Please do not forget to make a submission on the ods.ai with a link to this PR.