allow parallel fitting if multiple gpus

[pkienzle]

Bumps in sasview is currently single threaded. If multiple GPUs are available, then it should be run in parallel.

The complication comes when OpenCL is running on the CPU rather than the GPU, in which case only single threaded fitting should be used. For simultaneous fitting, mixtures or product models, part of the model may be run on GPU and part on CPU. This may not be an issue if OpenCL can manage multiple threads asking for OpenCL CPU resources at the same time, but that needs testing.