MVA friend trees from separate files (*_mva.root)

We propose the following changes to `plots.common` so that MVA values could be loaded from separate ROOT files that are placed next to the sample files, but have a `_mva` suffix.

The MVA trees (in TFile the tree `trees/MVA`) are added with AddFriend.

In the `plots.common.sample.get_file_list` function we filter out the `*_mva.root` files so that they would not be returned by the function.