UserTemplate + even/square crop sizes can lead to off-by-one errors

[matbryan52]

In testing #70, I had off-by-one errors when using a 3x3 mask with a central pixel == 1.

This is due to the hardcoded crop shape of (2 * search, 2 * search), so that the small mask is inserted into the search template off-centre.

Ideally, the crop shape should follow the mask shape for odd/evenness so that everything remains centered. Similarly there is no need to force the crops to be square, if the template is rectangular we can search in a rectangular window.

As part of #70 I've started to implement a backwards-compatible mechanism for a MatchPattern to supply a Tuple[int, int] from get_crop_size() which directly specifies the crop to search in.