Various Improvements

[varunagrawal]

1. Suppress maybe-uninitialized warnings from Eigen for Release build.
2. Fix some docstrings.
3. Use resetIntegration to initialize the ImuFactor and CombinedImuFactor classes.
4. Wrap ImuFactor2.
5. Added a new macro GTSAM_COUT which is for classes/types that don't have a .print() defined or have the << operator overloaded. An example usage is below:

Rot3 wRb = Rot3();
GTSAM_COUT(wRb);

which prints

wRB: 1, 0, 0  // Note that it also prints the variable name similar to GTSAM_PRINT.
0, 1, 0
0, 0, 1

Interestingly, there is no CombinedImuFactor2 which works on NavStates. This should be a TODO for us given the recent importance of NavState.