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.
