Minor filtering & make_kernel improvements

* Extremely minor tweak to filter.cpp -- the FilterDesc for 1D filters didn't probably set the table entries for the few filters that are fixed width.
* Change IBA::make_kernel to allow it to build 3D kernels (by taking a 'depth' parameter, which passing the default of 1.0 will make a 2D filter as before)
* Also allow make_kernel to request a "binomial" filter of a given width/height/depth, which is comprised of the binomial series weights (commonly used for image pyramid generation and such).