OpenCL support with PoCL tests

[klamike]

Adds support for OpenCL drivers, testing using the PoCL CPU backend. Closes #161.

[klamike]

Windows is failing due to

┌ Warning: Running at high integrity level, preventing OpenCL.jl from loading drivers from JLLs.
│ 
│ Only system drivers will be available. To enable JLL drivers, do not run Julia as an administrator.
└ @ OpenCL.cl C:\Users\runneradmin\.julia\packages\OpenCL\wTIUe\lib\cl\api.jl:165

which early cancels the rest of the github CPU jobs. So I am disabling the PoCL tests on the github windows CI.

[sshin23]

NVIDIA's failure seems to be that Percival is not reporting convergence to a stationary point. I think we can exclude OpenCL + percival. Not sure about intel. I think we can just exclude these caess and move on

[sshin23]

Thank you, @klamike!