Filter scenarios, alternatives and scenario_alternatives in scenario filter

[soininen]

This PR makes scenario filter to filter out scenarios, alternatives and scenario_alternatives that are unrelated to the selected scenario.

Resolves #206

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Base: 74.43% // Head: 74.49% // Increases project coverage by +0.05% :tada:

Coverage data is based on head (e1e0cbe) compared to base (c5620be). Patch coverage: 89.28% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #207 +/- ## ========================================== + Coverage 74.43% 74.49% +0.05% ========================================== Files 76 76 Lines 11739 11789 +50 Branches 2272 2273 +1 ========================================== + Hits 8738 8782 +44 - Misses 2554 2560 +6 Partials 447 447 ``` | [Impacted Files](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/207?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project) | Coverage Δ | | |---|---|---| | [spinedb\_api/db\_mapping\_base.py](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/207?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ19iYXNlLnB5) | `83.35% <77.77%> (-0.24%)` | :arrow_down: | | [spinedb\_api/\_\_init\_\_.py](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/207?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project#diff-c3BpbmVkYl9hcGkvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [spinedb\_api/filters/scenario\_filter.py](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/207?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project#diff-c3BpbmVkYl9hcGkvZmlsdGVycy9zY2VuYXJpb19maWx0ZXIucHk=) | `88.40% <100.00%> (+7.01%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project)

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