Tidy up permutations

The documentation was particularly bad at explaining the actual
concept implemented by class Permutation. Rename method "subset" to
"restrictTo" for consistency with the wording of the revised
documentation.
Element magnitude rescaling available to vectors as well as matrices

This is the first of a series of long overdue commits that will
make final the testing framework for system of linear equations.
Long overdue complete test matrix gen for LU decomposition, etc
Bug fix: multiplying by random orthogonal matrices from the right
Test for the reconstruction of a matrix from its LU decomposition

This reconstruction is used to test the goodness of the LU
decomposition. It is thus healthy to have an independent test.
Fix critical bug in testing of solving of system of linear equations
Small refactoring of dependencies between traits for LU decomposition
The classic left-to-right blocked LU decomposition

Just to compare with the less well-known recursive algorithm we
implemented first.
Variant of the classic blocked LU algorithm

Refactor some boiler plate in the benchmarks as well.