add RTS smoother for UKF

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.44% :tada:

Comparison is base (674bd83) 86.56% compared to head (92075dc) 87.01%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #114 +/- ## ========================================== + Coverage 86.56% 87.01% +0.44% ========================================== Files 14 14 Lines 1087 1124 +37 ========================================== + Hits 941 978 +37 Misses 146 146 ``` | [Flag](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/114/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/114/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | `87.01% <100.00%> (+0.44%)` | :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/114?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/114?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL3Ntb290aGluZy5qbA==) | `77.31% <100.00%> (+0.38%)` | :arrow_up: | | [src/ukf.jl](https://app.codecov.io/gh/baggepinnen/LowLevelParticleFilters.jl/pull/114?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL3VrZi5qbA==) | `96.51% <100.00%> (+0.89%)` | :arrow_up: |

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