Replace pocl by Intel OpenCL

[tbetcke]

I noticed serious problems with Pocl and haven't managed to stably run Bempp-cl with newer pocl versions. My suggestion is to replace pocl in all default build environments with the Intel OpenCL runtime. This is now available as part of conda, see

https://anaconda.org/conda-forge/intel-opencl-rt

In my experience this runtime works stably with Bempp.

[mscroggs]

I've tried installing intel opencl via conda and installing from the latest release deb files (https://github.com/intel/compute-runtime/releases/tag/23.09.25812.14) and in both cases I cannot get pyopencl to see intel opencl in the docker image.