Implementation of OpenVDB as a possible cache format for smoke

simulations.

This commits implements OpenVDB as an extra cache format in the Point
Cache system for smoke simulations. Compilation with the library is
turned off by default for now, and shall be enabled when the library is
present.

A documentation of its doings is available here: http://
wiki.blender.org/index.php/User:Kevindietrich/OpenVDBSmokeExport.

A guide to compile OpenVDB can be found here (Linux): http://
wiki.blender.org/index.php?title=Dev:Doc/Building_Blender/Linux/
Dependencies_From_Source#OpenVDB

Reviewers: sergey, lukastoenne, brecht, campbellbarton

Reviewed By: brecht, campbellbarton

Subscribers: galenb, Blendify, robocyte, Lapineige, bliblubli,
jtheninja, lukasstockner97, dingto, brecht

Differential Revision: https://developer.blender.org/D1721