add optional operating point in StateSpace

[baggepinnen]

Useful if the StateSpace system comes from a linearized nonlinear model.

[codecov[bot]]

Codecov Report

Merging #83 (aa23247) into main (db37027) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #83   +/-   ##
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  Coverage   68.59%   68.59%           
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  Files          23       23           
  Lines         952      952           
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  Hits          653      653           
  Misses        299      299           

Impacted Files	Coverage Δ
src/Blocks/continuous.jl	98.17% <100.00%> (ø)

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[baggepinnen]

Why x_start but u0 and y0?

x_start denotes a start value, whereas x0, u0 denote nominal/operating points (a rather commonly used notation to describe the operating point for delta-coordinates $\Delta x = x - x_0$).