Adjusting SceneCache read time in Houdini.

We need to adjust the time we read SceneCaches inside Houdini because Houdini treats time starting at Frame 1, while SceneInterfaces treat time starting at Frame 0. Note that we should really be adjusting time by 1/fps used at cache time, but we don't have access to that, so we currently assume that Houdini's current frame rate is representative.
HoudiniScene uses SceneInterface standards for time.

Calls to HoudiniScene will expect SceneInterface time (starting from Frame 0), and will internally adjust when querying Houdini.
SceneCache ROP adjusts time before querying HoudiniScene
Correcting HoudiniScene customAttributeReaders so the ReadFn accounts for time.

Now all links written from Houdini are time re-mapped.
defaultTime is now private and renamed to adjustedDefaultTime
Merge pull request #132 from andrewkaufman/timeOffset

Fixing SceneInterface Time in Houdini