Unify channel index type in nth_channel_view and image_view::num_channels

### Actual behavior

- `image_view::num_channels()` returns `size_t`
- `nth_channel_view(View, int n)` expects `int` for index

Apart from signed and unsigned mix-up causing the annoying compilation warnings,
if no rationale of the mix is provided, it's a sloppy design of the interface.

### Expected behavior

Use of common type for indexing of `image_view` channels.

### C++ Minimal Working Example

```cpp
#include
namespace gil = boost::gil;
int main
{
gil::rgb8_image_t img;
auto v = gil::view(img);
for (std::size_t i = 0; i < v.num_channels(); i++)
auto _ = nth_channel_view(v, i);
}
```