allow multiple measurement models in Kalman filters

[baggepinnen]

This makes it possible to use

• More than one measurement model, useful for sensor fusion.
• Use a linear measurement model with a nonlinear filter.

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 19 lines in your changes missing coverage. Please review.

Project coverage is 91.28%. Comparing base (5a7a675) to head (0a3b1bf).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
src/measurement_model.jl	87.64%	11 Missing ⚠️
src/ukf.jl	96.12%	5 Missing ⚠️
src/utils.jl	50.00%	2 Missing ⚠️
src/ekf.jl	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #171      +/-   ##
==========================================
+ Coverage   91.21%   91.28%   +0.07%     
==========================================
  Files          16       17       +1     
  Lines        1446     1607     +161     
==========================================
+ Hits         1319     1467     +148     
- Misses        127      140      +13     

Flag	Coverage Δ
unittests	91.28% <93.75%> (+0.07%)	⬆️

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