Add yet another option for how to run the check_whitespace hook: incremental-with-patch. The idea is: If a file was clean, block commits that make it unclean. If it wasn't clean, don't block, but still propose a patch to clean it up. This may not work for folks using an hg gui. Thanks to Fred for the suggestion.

Removed some unneeded cleanup code left over from previous attempts to fix the segfault-on-exit problem.

Fixed BLAS compilation instructions under Windows to be more coherent with other Windows instructions

Changed gradient for tensor.Prod to account for special cases where inputs contain zeros. An intermediate (very slow) version is in comment, to be removed in next commit.