* Hide convolve_1d and convolve_2d in namespace detail
* Hide kernel_2d in namespace detail.
* Tidy up naming of horizontal/vertical to y/x for kernel center.
Following @lpranam comments on planned changes to convolve_2d,
which will not be ready for Boost 1.72,
@mloskot proposed to hide the function as implementation detail,
https://lists.boost.org/boost-gil/2019/10/0316.php,
until it is ready and @stefanseefeld agreed on the proposal,
https://lists.boost.org/boost-gil/2019/10/0320.php.
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.
First stab at convolution tests includes cases with the identity kernel.
Move `pixel`-s streaming facility used by Boost.Test (required by
`BOOST_TEST` macro) to the common header for re-use in other tests.
