PytatoPyOpenCLArrayContext: don't trust the arg limit reported by the GPU (#198)

* PytatoPyOpenCLArrayContext: don't trust the arg limit reported by the
GPU

Apparently, CUDA doesn't like it when argument sizes get close
to the reported limit.
Avoids PTX/JIT errors of the type
CUDA_ERROR_INVALID_IMAGE: device kernel image is invalid
CUDA_ERROR_FILE_NOT_FOUND: file not found

* restructure and clarify

* add warning, clarify comment

* add comment about device naming