Handle measurement models which return group, but not the Eigen vector.

[slovak194]

Dear @artivis ,

I follow your code here: https://github.com/artivis/kalmanif/blob/5e0f1f2275e22979afa39d8f67782929f21c1072/include/kalmanif/impl/unscented_kalman_filter_manifolds.h#L287

I see so far that the result from the measurement model is expected to be a vector. Are there any thoughts regarding the support of measurement models which return group? Particularly, when UEKF is used to fuse several high level sources of orientation expressed as SO3 members.

Here are my thoughts so far:

1. Use a container to store sigma points instead of columns in the matrix (std vector should be fine).
2. Use proper averaging and weighting methods to compute mean and covariance on manifold (average_biinvariant?).

I think I'll be able to contribute with a little help from your side.

Best regards, Alex