Assistant should provide clear warning when pyopencl not installed #67
Description
I've been setting up python/napari on a new machine, creating an environment with conda, and then pip installing various napari plugins including pip install napari-pyclesperanto-assistant (mostly to grab all its dependencies).
Using napari or naparia in the command prompt gives the following message on the next line "Assistant skips harvesting pyclesperanto as it's not installed." Checking packages, it certainly was correctly installed.
By viewing tracebacks from my plugins which depend on pyclesperanto, I was able to determine the actual error was lack of pyopencl.
I suggest either adding pyopencl as a dependency to pyclesperanto, or changing the error message for napari-assistant (though this is less targeted at the source).