Add MASTR based OEDS loader - move scenario loaders to separate folder

[maurerle]

This adds the documented OEDS loader which can be used to create scenarios from MASTR for NUTS locations, if an open-energy-data-server is available.

The scenario loaders are moved to a separate scenario folder and all begin with loader_

Documentation is added accordingly, which reflects the possibilities and a small usage guide of it

[codecov[bot]]

Codecov Report

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

Comparison is base (b3aa9c1) 78.49% compared to head (cc1a101) 80.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #264 +/- ## ========================================== + Coverage 78.49% 80.79% +2.30% ========================================== Files 39 38 -1 Lines 4260 4135 -125 ========================================== - Hits 3344 3341 -3 + Misses 916 794 -122 ``` | [Flag](https://app.codecov.io/gh/assume-framework/assume/pull/264/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework) | Coverage Δ | | |---|---|---| | [pytest](https://app.codecov.io/gh/assume-framework/assume/pull/264/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework) | `80.79% <100.00%> (+2.30%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=assume-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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