FEATURE: Ability to set a `max_displacement` for u and v

[rhysgt]

It seems that max_displacement needs to be less than the smallest image dimension.

For optical DIC images, we normally crop the images down to roughly the gauge size (to save disk space), but this then limits our max_displacement to be smaller than the strain applied to our sample.

This could be fixed if we could select a different max_displamcent for x and y, instead of a single value.