update calculation of outErr

Ok. I think I have the hang of this pull request thing.
I have 2 compile errors, as you'll see.
1) The compiler claims that there isn't a version of InvertCramer for a MPlexHS
2) The compiler says that "-" is not overloaded for MPlexLS - MPlexLS, so either you have some other way you want to do it (which would probably be good because it would get rid of a temporary), or that operator needs to be overloaded.