Add Gaussian Mixture residuals to batch

[vkorotkine]

Implemented Gaussian Mixture Model (GMM) residuals for batch. Useful for unknown data associations and loop closures, for instance. Based on the papers below.
A TODO is to add an iterated EKF that uses batch residuals so that these may be used in a filtering context.

@Article{9381625,
author={Pfeifer, Tim and Lange, Sven and Protzel, Peter},
journal={IEEE Robotics and Automation Letters},
title={Advancing Mixture Models for Least Squares Optimization},
year={2021},
volume={6},
number={2},
pages={3941-3948},
doi={10.1109/LRA.2021.3067307}}

@misc{korotkine2024hessian,
title={A Hessian for Gaussian Mixture Likelihoods in Nonlinear Least Squares},
author={Vassili Korotkine and Mitchell Cohen and James Richard Forbes},
year={2024},
eprint={2404.05452},
archivePrefix={arXiv},
primaryClass={cs.RO}
}