Interpolation refactoring

This is among other small things a refactoring of the interpolation

• Make textures less special (now subclasses of interpolator)
• Remove TextureCache
• (Ab-) use finite differences to enable spatial derivatives of interpolators
• enable code to use https://github.com/theHamsta/CubicInterpolationCUDA (not included into pystencils, not even as a submodule)

I can use it to transform Lenna and get gradients with Torch wrt. to image content, x-translation, y-translation

Also a nice image lenna