arclengths plot broken for hessian PDFs

Created by: wilsonmr

as discussed this commit led to a dimension mismatch for hessian PDFs in arclength calculation

https://github.com/NNPDF/nnpdf/pull/831/commits/74f710e84f80a03737e972f009270bcfc3edcc4f

To be quite honest it's very understandable considering our proprietary function for getting number of replicas does a different thing for MC replicas without much documentation. Probably this should go somewhere