Remove Submatrix and Submatrixer

Submatrix(a, i, j, r, c) is trivially implemented as m.View(a, i, j, r, c); m.Clone(m),
and we do not provide the Copier equivalent.
Merge pull request #23 from gonum/purge

Remove Submatrix and Submatrixer
Return ErrSingular for singular matrix solves
Add tall and wide tests for singularity
Merge pull request #32 from gonum/solve

Return ErrSingular for singular matrix solves
Merge remote-tracking branch 'upstream/master'

Conflicts:
	mat64/dense.go
took into account the few doc remarks
benchmarking the two versions of IsSymmetric()
remove the benchmarks
added the slice argument to Diag()