Bug -- ExposureContrastTransform: imperfect inverses in "log" mode

We discovered that inverse "log"-style ExposureConstrastTransforms do not seem to behave as expected when certain parameters are set to non-default values

For example:
```
# In "Log" mode, when setting both *exposure* AND *logExposureStep* to
# non-default values, transform inverses do not cancel out (when applied as discrete processors)

xf = ocio.ExposureContrastTransform(
style=ocio.EXPOSURE_CONTRAST_LOGARITHMIC,
exposure=-2.0,
logExposureStep=0.0571
)
```

Here's a notebook that demonstrates what I mean:
https://gist.github.com/zachlewis/7b2bfc1f2fae115a32bf2232b7515442