* 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.
