Clean code and add some docstrings.
Add stub unit tests for image rescaling.
Add static method to cast pixel to another type.
Add min, max and zero pixel value.
Add unit tests for `PixelTraits` class.
Rename template parameter `Layout` as `ColorSpace`.
Replace `color_min_value` by `PixelTraits<T>::min` and so on.
Rename `Layout` as `ColorSpace`.
Remove old code since it is used to test `PixelTraits`.
Add additional test for interpolation.
Fix unit test in channel conversion.
Fix `convolve_array`.
Fix compilation issue in `Subimage` module.
Cast channel in interpolation and rescaling instead of channel conversion.
Add unit tests for `reduce` functions.

This is not very robust but it is better than nothing.
Fix unit tests for linear interpolation.
Add other test for linear interpolation.
Add missing `template` keywords.
Merge pull request #144 from DO-CV/maint-test-image-scaling

[MAINT] Add unit tests for image rescaling.