Stochastic volume interpolation

The tricubic interpolation mode of AiVolumeSampleXXX() calls, such as those used by the standard_volume shader, now uses a stochastic method of interpolation that greatly reduces the amount of voxel data read in each sample call. This can speed up rendering by as much as 20% in assets such as the WDAS cloud data set. Note that this technique does not always produce a perfectly matching image as a result, and can be toggled via the stochastic_volume_interpolation render option (default is on). (ARNOLD-3856)