* 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.
Use of `std::extent` was introduced in PR #200 but it turns out as
not applicable for `std::array` or derived types (e.g. `kernel_1d_fixed`).
This led to obtaining invalid size of `kernel_1d_fixed` and erroneous
results of the rows and columns convolution.
This change replaces `std::extent` with new public constant member
`kernel_1d_fixed::static_size`.
Since `kernel_1d_fixed` is derived from `std::array`, the Alternative
could be to use `std::tuple_size` specialization for `std::array`.
Add static assertion to require that kernel size must be odd to
ensure validity at the center.
