Export the structure of a problem to the public API

This adds three new public methods to ceres::Problem:

  Problem::GetResidualBlocks()
  Problem::GetParameterBlocksForResidualBlock()
  Problem::GetResidualBlocksForParameterBlock()

These permit access to the underlying graph structure of the problem.

Change-Id: I55a4c7f0e5f325f140cb4830e7a7070554594650