Updated Houdini Exporters to Python 3 + invert X component (#27)

* Updated PCache Exporter to Python 3 + invert X component

* Implemented X Inversion in VF Exporter + upgraded to python 3

* Final Harmonization between TGA/VF for SDF and Vector Fields inversions

* Updated Changelog

Co-authored-by: Thomas Iché <thomasi@unity3d.com>

CHANGELOG.md

 
 * Added support for Texture2D Arrays
 * Fixed ImageSequencer for 21.1 and newer versions
+* Houdini Exporters: Upgraded to Python3
+* Houdini Exporters: Added X Axis Inversion options to match Unity's 3D Space 
 
 ## [2.0.0-preview] - 2020-03-23
 
 
 ## [1.0.0-preview] - 2019-06-28
 
-* Initial Version
+* Initial Version