Remove deplicated parameters in ieSessionGet
Add gpu-computing parameter to vcSESSION
Fix typo

It should be oiGet, not sceneGet, I think.
Allow for gpuArray input

1. gather() will convert gpuArray to double and will do nothing if the
input is already double
2. when we do type checking, we should allow gpuArray input type
Modify gpu compute code for oiCompute

1. Remove the old stand-alone code for gpu computing. It's ugly and
dangerous. Now, gpu computing and cpu computing use same routine.
2. Add some gpu compute conversion in customOTF
3. To run the code successfully, we need the input scene.data.photons to
be a gpuArray. Currently, sceneCreate and sceneFromFile haven't been
changed for this purpose...Thus, it could fail if not set
scene.data.photons to gpuArray manually at this point...