Add pedestal to Grating stimulus

[cafarm]

Grating should have a pedestal or offset that makes it easy to shift it's mean value.

[cafarm]

I haven't decided how to handle this. Should the default pedestal be 0 or 0.5? Should it be RGB or a single intensity? I'll have to think about it more. For now, it's easy enough for users to create their own grating however they like by using the Image stimulus.