add GaussNewton parameter-estimation example

[codecov[bot]]

Codecov Report

Patch coverage: 20.00% and project coverage change: +1.51% :tada:

Comparison is base (4f88d8e) 85.05% compared to head (d3788b1) 86.56%. Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #111 +/- ## ========================================== + Coverage 85.05% 86.56% +1.51% ========================================== Files 17 14 -3 Lines 1198 1087 -111 ========================================== - Hits 1019 941 -78 + Misses 179 146 -33 ``` | [Flag](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/111/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/111/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | `86.56% <20.00%> (+1.51%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/111?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [src/smoothing.jl](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/111?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL3Ntb290aGluZy5qbA==) | `76.92% <14.28%> (-8.80%)` | :arrow_down: | | [src/PFtypes.jl](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/111?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL1BGdHlwZXMuamw=) | `82.99% <100.00%> (+0.11%)` | :arrow_up: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/111/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson)

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