Add supporting code for GPU-based ops

We want to make implementing GPU-based ops as easy as possible. The glue code to execute GPU-based processing from Java is usually the same. The two main flavors to consider supporting are OpenCL and CUDA.

We can start by implementing a couple of GPU-based ops, and then factoring out common code into a shared type hierarchy. Due to the addition of dependencies for working with OpenCL and/or CUDA, we will likely need to create a new `imagej-ops-gpu` project (and/or `imagej-ops-cuda` and/or `imagej-ops-opencl` projects) which extend `imagej-ops`.