Reduce repository size

[fmaussion]

A git clone on cosipy is about 280 mb large, which is a lot for a git repository,

There is some data in the data and docs folders (27M and 18M), but most of this size is taken by the .git folder. I think that it would be beneficial to the repository to:

• delete all unused branches
• if this is not enough, you might want to squash all commits related to data (this is more involved and requires force pushing to master)