Added downbeat switch to beat evaluation

[flokadillo]

If downbeat flag is true, downbeats instead of beats are read from file.

[superbock]

Instead of using the "update branch" button it's better to rebase the whole branch on top of the develop branch. To do so, please follow these steps:

git checkout develop
git pull
git checkout feature/downbeat_eval
git rebase --onto NEWBASE OLDBASE

with NEWBASE being the latest commit on develop and OLDBASE the parent of your first commit in this branch. I'm not sure whether develop can be used as NEWBASE, probably yes.

If everything is ok, you can force push this branch.