Add interfaces to other imaging libraries

[crisluengo]

In DIPlib we can, in a simple header file:

• define a function to create a dip::Image object around image data from other libraries,
• define a function to convert a dip::Image to an image of that library (either as a view over the same data segment, or by copying the data),
• and optionally define a dip::ExternalInterface so that DIPlib functions, when allocating a data segment for an image, can create an image of the other library underneath.

We currently have this for OpenCV and Vigra. We could add ITK and/or SimpleITK, and any other library that people would find useful to combine with DIPlib.

[crisluengo]

It might be useful to add an interface to Eigen.