Add switchable constraints

[mikeferguson]

Two papers describe this:

• Towards a Robust Back-End for Pose Graph SLAM has the better description of the algorithm, although it uses the sigmoid, which:
• Switchable Constraints for Robust Pose Graph SLAM says to use [0,1] range, not sigmoid.

The infrastructure is already there for determining if a particular edge in the graph is switchable or not (i.e. was it made from odometry or global scan matching)