Removed unused class members.

* Focal length calculation now done with FOV data from OpenNI and not cached.
Fixed Clang errors.
Simplified OpenNI library search paths.
Fixed abstraction around existing OpenNI 1.x grabber.

This should help with switching between OpenNI 1.x and 2.x grabbers.
Added fall-through to OpenNI 2 grabber.

If OpenNI 2 is being used, have openni_grabber.h include openni2_grabber.h.
Enabled kinfu CMake option with OpenNI 2.
Added OpenNI2 library checks when building kinfu app.
Disabled some kinfu classes that depend on OpenNI 1.x when OpenNI 2 is being used.
Masked oni_grabber errors when the OpenNI 1.x wrapper is not used.
Added conditional compile instead of changing OniGrabber cast type.

Should prevent regression problems when using OpenNI 1.x.
Disabled pcl_record_tsdfvolume when using OpenNI 2.
Refactored obscure kinfu app function names.
Fixed confusing names in kinfu app.
Added getter for image step (row stride).
Removed get[..]Stream() calls and added triggerSingleFrame(..)
Added single-frame triggering.
Split kinfu operator(..) into trackFrame(..) and integrateFrame(..)
Merged shutdown(..) with ~OpenNI2Device(..)
Fixed Clang warnings.