Rename `Switch` to `Mixture`

[bartvanerp]

Addresses https://github.com/biaslab/ReactiveMP.jl/issues/262.

Perhaps let's wait with merging, because of recently submitted demo.

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Codecov Report

Base: 79.70% // Head: 79.70% // No change to project coverage :thumbsup:

Coverage data is based on head (602b9f3) compared to base (538f335). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ======================================= Coverage 79.70% 79.70% ======================================= Files 11 11 Lines 1163 1163 ======================================= Hits 927 927 Misses 236 236 ``` 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=biaslab). 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=biaslab)

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