Dispatching Python covariance matrices
Created by: siranipour
We now dispatch covmats purely from python. Addresses various points including https://github.com/NNPDF/nnpdf/issues/1476#issuecomment-979823687 and https://github.com/NNPDF/nnpdf/issues/1475#issuecomment-979425362
Let's see what the tests do