Refactor Strategies to Recommenders

[Scienfitz]

• move strategies to recommender module
• restructure recommender module to have .meta and .pure for subpackages
• new class MetaRecommender from which old strategies now derive, their name has changed from <xyz>Strategy to <xyz>MetaRecommender
• deprecations for old strategies
• rename Recommender to PureRecommender in analogy to MetaRecommender
• rename NaiveHybridRecommender to NaiveHybridSpaceRecommender
• refactor tests
• refactor examples
• reduce transfer learning example parameters during SMOKE_TEST

NOT included

• Followup PR1: MyPy activation for recommenders
• Followup PR2: user guide files in this PR can be mostly ignored, I have mainly fixed references in examples and user guides, but also their structure needs an overhaul (eg strategies don't exist anymore)

[Scienfitz]

@AVHopp unfortunately a lot of files have changed event hough this is just a restructuring/renaming, but we need to prioritize this PR

[AVHopp]

Main things I spotted:

• At several places, we actually need the word strategy, e.g. when speaking about the abstract concept of a strategy ("DoE strategy") instead of the actual object.
• In hypothesis related files, we also need to keep the word strategy.
• There are some places where I'd prefer some more explanation regarding the new naming. This is nothing too big, with the exception of the "strategy" user guide where we should add some note saying that the names do not match currently although the functionality remains as described.

[AVHopp]

Just one minor question, other than that this is gucci now. Thanks for all of the tedious work :)