HybridFactorGraph discreteKeys has duplicate keys

[varunagrawal]

Description

Calling HybridFactorGraph::discreteKeys results in duplicate keys which can lead to subtle bugs.
For example, if we pass in the result to gtsam::DiscreteValues::CartesianProduct(); this would result in a much more expensive computation since instead of 3 discrete keys, we now have many more.

We can't use discreteKeySet since that method throws away the cardinality of the discrete keys.

Steps to reproduce

Call HybridFactorGraph::discreteKeys and print out the result.

[dellaert]

Good catch, fixed in #1380, please approve that ASAP as I want to build on it.