LQG overhaul

[baggepinnen]

This PR modernizes the LQG interface slightly

[coveralls]

Coverage decreased (-1.7%) to 55.582% when pulling f42be61 on LQGupdate into 3103ce9 on master.

[coveralls]

Coverage increased (+0.1%) to 57.355% when pulling f42be61 on LQGupdate into 3103ce9 on master.

[codecov]

Codecov Report

Merging #332 (3a14764) into master (a8745f3) will decrease coverage by 0.01%.
The diff coverage is 84.37%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
- Coverage   82.89%   82.88%   -0.02%     
==========================================
  Files          31       31              
  Lines        2859     2857       -2     
==========================================
- Hits         2370     2368       -2     
  Misses        489      489              

Impacted Files	Coverage Δ
src/ControlSystems.jl	100.00% <ø> (ø)
src/types/lqg.jl	83.00% <84.37%> (-0.34%)	⬇️

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