Passing data among plugins

Data exchange among multiple plugins can be carried out in *ImagePy* in [different ways](https://forum.image.sc/t/rag-thresholding-in-imagepy/34128/4?u=zcs). As an example, see [this](https://forum.image.sc/t/rag-thresholding-in-imagepy/34128/6?u=zcs). Those solutions are better than the currently used module-level global object.
Also see the related issue #7.