Fix errors with singular matrix in CoxPH
Fix errors with singular matrix in CoxPH

This fixes the error of singular matrices when solution
is 'too exact'.

Signed-off-by: Jonas Kalderstam <35a2c6fae61f8077aab61faa4019722abf05093c@kalderstam.se>

nerdless/lifelines,jstoxrocky/lifelines,CamDavidsonPilon/lifelines,wavelets/lifelines