Un-initialized Eigen matrix (frame covariance)

When calling the function Eigen::MatrixXs Problem::getFrameCovariance(FrameBasePtr _frame_ptr) (problem.cpp l.506), the returned Eigen matrix may not be initialized if the function ceres::Solver::Summary CeresManager::solve() has not been called before.

Not sure if it is an intended behavior but it implies synchronization issues between the calls of the two aforementioned functions that is not easily/clearly 'checkable'.

Propositions:

• Initialize the frame covariance matrix w.r.t the main odometer integrated covariance.
• A clear statement that the frame has been optimized hence its covariance matrix is initialized. E.g. frame_ptr->hasBeenOptimizedOnce() returns a bool, and/or frame_ptr->optimizationCounter() returns an int incremented every times the frame goes through the solver.