refactor MDP.computePR() code to do with rewards matrix to be more clear
refactor MDP.computePR() code to do with rewards matrix to be more clear

sawcordwell/pymdptoolbox,silgon/pymdptoolbox,yasserglez/pymdptoolbox,McCabeJM/pymdptoolbox,sawcordwell/pymdptoolbox,silgon/pymdptoolbox,McCabeJM/pymdptoolbox,yasserglez/pymdptoolbox