Add true determinant to QR and its variants

What does this implement/fix?

Fixes #471 (closed). Implements determinant() method which gives true determinant, for HouseholderQR, ColPivHouseholderQR, FullPivHouseholderQR, CompleteOrthogonalDecomposition. Documentation and test code is included.

Additional information

To calculate determinant of Q matrix, struct householder_determinant is added into namespace internal. This class is specialized for real scalar type, so that it can make use of the fact that each reflection negates determinant for real matrices.