Fixed arbitrary phase in MDS

I added a few lines to set to a predictably value the
arbitrary phase in the eigenvectors used for MDS.
This should make the runs more reproducible.
Fix arbitrary phase in eigenvectors in diagMat

As suggested by Gareth, I moved the fix in arbitrary phase from
ClassicalMDS to diagMat
Fixed regtest after last commit
Merge branch 'fix-mds' into v2.1