Minor convolution fixes

Minor fixes to convolution, including removing docs for nonexistent parameters and adding a new sanity check to prevent convolution of enormous arrays that are almost certain to fill swap on typical machines, especially laptops.  A new parameter allows overriding the restriction.

TODO: add a test for the hugeness prevention.  What Exception class should I use in place of the anonymous Exception I have now?

@astrofrog, you should probably review this (should be easy)