replace R with q

[ducha-aiki]

The R is defined as a getter function above, so one cannot assign values to R itself, and attempts to do so breaks clang compilation.

Another functions already are setting q correctly instead: https://github.com/danini/graph-cut-ransac/blob/master/src/pygcransac/include/relative_pose/bundle.cpp#L227 So this PR just makes it consistent.