[PATCH 0 of 4] Permutation matrices: One bugfix, one enhancement, sparse-perm ops, and a use in lu().

[Jason Riedy]

The bugfix:
  * Make the column permutation vector in sparse LU cols()-long.

The simple enhancement:
  * Add an override to Octave's find() for permutation matrices.
Seems odd that find takes O(n^2) time on permutation matrices.  IIRC,
it's the only operation to convert a perm. matrix to a perm. vector.

The operations:
  * Add perm * sparse, perm \ sparse, sparse * perm, and sparse / perm operations.
There might be a way to use the existing indexing ops, but I couldn't
see it.

And a future use in lu:
  * sparse-perm-ops Have sparse LU return permutation matrices rather than sparse matrices.
This last one is included mostly because it flushed out the bug, but
it is a thought for the future.

Jason