GPU Computing

This pull request is mainly for some update for gpu computing in ISETBIO. It consists of following part:
1. add 'gpu compute' as a parameter of vcSESSION. User could now turn-on / off gpu computing by ieSessionSet('gpu compute', true/false);
2. Remove the stand-alone code for gpu computing for oiCompute (actually customOTF and opticsOTF). Now  GPU computing and CPU computing use the same code. The only difference is matrix type (GPU - gpuArray, CPU - double/single matrix)
3. If we do gpu computing, the scene we created will be on GPU. The oi data will be on GPU. The sensor data will be on GPU. Thus, it's not like previous version where GPU computing only took place in oi compute.
4. Add routines to convert gpu object (scene/oi/sensor) back to main memory. This function is used very time we call vcAddObject and vcAddAndSelectObject to avoid potential GUI issues. If the object is already in main memory, nothing will happen.

GPU computing requires Matlab 2014a and CUDA enabled machine. Maybe it's hard for some people to test or use, but I guess we would like it some day.

Best,
HJ