Add method to return covariance in tangent space

This CL is required to build Tango.

Inspired by this commit in RedwoodInternal repository:
  commit 09dde53c248e04f432b5eccceea5daeedb706aea
  Author: Mike Vitus <mike@hidof.com>
  Date:   Wed Apr 23 11:05:17 2014 -0700

Change-Id: I328b6634969de4ccdd71947945aa67a49ee9073f