Renaming HouseholderSolve to LeastSquares and adding documentation for the Kahan matrix

--HG--
rename : examples/lapack-like/HouseholderSolve.cpp => examples/lapack-like/LeastSquares.cpp
rename : include/elemental/lapack-like/HouseholderSolve.hpp => include/elemental/lapack-like/LeastSquares.hpp