Add support for bar based guessing

[xstasi]

This commit implements my suggestion in Issue #5.

The bar detection is manual but it works for my use case, if necessary the automatic detection can be added later on.

[xstasi]

Hi there!

I have made a number of changes to the code in order to fit my use case, which is enhancing midi data with chord information for the purpose of ML data set building.

The default behaviour should be unchanged as my changes are all optional, so the code that used autochord before should be unaffected.

Feel free to incorporate my changes if you want, otherwise I'll keep using my fork :) completely up to you!

Cheers and thanks for starting the project!