DOC: Add an image example for convolve2d docstring,
using a small kernel with a large image, and showing that it can do complex input.  TODO: Is there a way to turn off the axis for all 3 subplots at creation?
DOC: Add a 2D cross-correlation example.
Currently this is slow, but the template needs to be large or it won't match reliably. Normalized cross-correlation would probably work better, but then it wouldn't be a demo of correlate2d...
DOC: Add an example of 1D cross-correlation
Use it to implement a matched filter, inspired by https://en.wikipedia.org/wiki/Matched_filter#Example_of_matched_filter_in_digital_communications
DOC: Add 1D and 2D fftconvolve examples
DOC: separate plotting code in convolve examples
Merge pull request #3553 from endolith/convolve_examples

DOC: Convolve examples