Remove OpenEXR from requirements and warning #421

The docs and code say that OpenEXR is an optional dependency (

or here but is seemingly is not. This means I cannot install the TFG package, since the OpenEXR does not get installed properly. Furthermore, the warning is rather annoying, as we would like to use some simple functionality from the package (bsplines) in a completely different context.

I would propose to
a) remove OpenEXR from the dependency list and make it optional (as documented in docs and code)
b) remove the warning and instead make OpenEXR a local import inside the function. If the function is called and it cannot be imported, it fails saying that it has to install OpenEXR first.

What do you think? Glad also to make this changes and contribute