Calculate covariances for parametric tree solve

[Affie]

It will not be implemented in PR #468 and is needed to compete the tree solve.

[Affie]

Covariances are calculated using the inverse of the Hessian matrix from Optim.
It still needs some work on robustness.

[Affie]

This now has to be updated to be relevant on manifolds, see https://github.com/JuliaRobotics/IncrementalInference.jl/blob/experimental/21Q2/mani_optim/examples/dev/optim_sandbox.jl as starting point