Improvements to convolve

convolve_util now automatically deals with even/odd kernel shapes by forcing the kernel size to an odd number (it does not change the with of the box or the gaussian, just the kernel image).

Also allows for the fft version to be used, which would be useful for big kernels.  However, I haven't propagated that change elsewhere - the fft version is inaccessible to, e.g., `show_grayscale` at the moment.