Develop a plan to deploy specific Covariance Matrix representations

[tfoote]

This is a follwup to the pre Foxy Message API review

Create new Covariance representation (3x3 upper triangle and 6x6 upper triangle) as well as potential helper functions. Identify areas to leverage it and then determine a migration path. New messages can leverage it, but existing ones will have a long migration.

They can likely be also reduced to floats to cut the size of the message as well.