Add GNKI to the toolkit

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Issue

New tool to complement EKI,EKS, etc. based on Gauss-Newton and Statistical Linearization, from here
We will use the terminology GNKI, rather than the IEKF-SL terminology from the paper

Solution

Tasks

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1. Implement the IEKF tool, #405
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2. Add unit testing for IEKF in the framework of other tools #406
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3. Add IEKF as a comparison into at least one example #424
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4. Add a docs page describing setup, and API. Add derivation as with other EKP tools #415
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5. Add functionality to impose the prior for EKI/ETKI to enable consistent comparison between processes