Support properly OpenEXR

[edgarriba]

the crate image-rs supports openexr images so that we can import/export floating point images. However, the cv::Tensor class assumes for now only byte representation.

[carlosb1]

https://github.com/cessen/openexr-rs/blob/master/examples/simple_input.rs - can be done something like this to import images to tensor?

[edgarriba]

With the current image-rs we already can decode exr, however we need to somehow redesign the cv:: Tensor class so that can hold not only uint8 but also floating point data.

[gau-nernst]

I think using Rust generics might work? I can try when I have some time.