add option to load subset

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As proposed in #24 (comment)

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ToDo:

• If we work on a subset and run the drum extraction of the midi files we will also overwrite the already fully one with the subset one - one of the examples why the state management in jupyter is horrible because its all over the place, spammed with global variables. The problem is that caching on a subset is even more difficult because the subset must be fixed in order that the caching works as expected. Maybe instead of storing the stuff in memory we should write a metadata file but this is also not so nice... mh

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Obsolete with #33