add option to design observers with direct term

[baggepinnen]

This applies to several functions

• kalman
• observer_controller
• place
• observer_filter (new function in this PR)

This PR also adds the option to let observers output the state estimate rather than the output estimate.

The reference CCS refers to Computer Controlled Systems by Åström and Wittenmark

[JuliaControlBot]

This is an automated message. Plots were compared to references. 11/11 images have changed, see differences below. After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here. Difference	Reference Image	New Image
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:heavy_check_mark: 0.011
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:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
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[JuliaControlBot]

This is an automated message. Plots were compared to references. 11/11 images have changed, see differences below. After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here. Difference	Reference Image	New Image
:x: 0.047
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017

[codecov[bot]]

Codecov Report

Merging #861 (b914815) into master (a047432) will increase coverage by 36.19%. The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #861       +/-   ##
===========================================
+ Coverage   56.53%   92.72%   +36.19%     
===========================================
  Files           5        5               
  Lines         329      330        +1     
===========================================
+ Hits          186      306      +120     
+ Misses        143       24      -119     

see 3 files with indirect coverage changes