pyopencl missing from dependencies

This may be as simple as
`pip install pyopencl[pocl]`

but it seems like there are some complexities when running from within a conda environment that also need to be documented.

See [opencl installation notes](https://documen.tician.de/pyopencl/misc.html#using-vendor-supplied-opencl-drivers-linux) and [PLATFORM_NOT_FOUND_KHR issue](https://github.com/conda-forge/pyopencl-feedstock/issues/32)