Request for API to calculate the value of setLogExposureStep

[jsbache]

When creating an exposure transform for a logarithmic color space we get the best (most predictable) behavior if we use a an exposure step that reflects the colorspace. The exposure transformation exposes this API: "setLogExposureStep". However, given an arbitrary color space there is no (easy) way of calculating this value. We request that an API such as defaultLogExposureStep is exposed on an OCIO color space.