Metis ordering backend supports only squared matrices
Add matrixR() to get the triangular factor from the Householder QR
Correct the SPQR backend for rank-deficient matrices and delete some public functions
Add setPivotThreshold to Sparse QR
Set built-in sparse QR as the default sparse solver  and add ComputationInfo for Levenberg Marquardt,
Be more explicit about user-defined functions in Map tutorial.
See discussion on mailing list on 18 + 19 Feb 2013.
Guard against transposeInPlace on non-square non-resizable matrix.
Inspired by question by Martin Drozdik at stackoverflow.com/q/14954983