Align with `subsampled_view` overloads accepting `point_t` and `coord_t`
Both types are based on `std::ptrdiff_t`. Fixes compilation warnings.
Tidy up `subimage_view` and `subsampled_view` with trailing return.
Add tests for `subimage_view` in core and dynamic_image extension.
(First tests for the dynamic_image, hurray! :))
Add explicit cast to double in division operator for point<T> to
fix compilation warnings.
