Add dlqr method for StateSpace input

[adrhill]

This method was already documented in the dlqr docstring but not implemented:

ControlSystems.jl/src/synthesis.jl

Line 67 in 6c54a25

"""`dlqr(A, B, Q, R)`, `dlqr(sys, Q, R)`

[baggepinnen]

LGTM. Later on we can probably remove the d versions of these methods since now the Continuous/Discrete is part of the type and this can be solved with dispatch rather than separate functions.

[codecov]

Codecov Report

Merging #406 (f976876) into master (6c54a25) will decrease coverage by 5.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   82.40%   77.12%   -5.28%     
==========================================
  Files          31       31              
  Lines        2824     2837      +13     
==========================================
- Hits         2327     2188     -139     
- Misses        497      649     +152     

Impacted Files	Coverage Δ
src/synthesis.jl	67.64% <100.00%> (+2.02%)	⬆️
src/pid_design.jl	32.57% <0.00%> (-45.38%)	⬇️
src/discrete.jl	40.54% <0.00%> (-43.72%)	⬇️
src/types/SisoTf.jl	25.00% <0.00%> (-25.00%)	⬇️
src/types/SisoTfTypes/SisoZpk.jl	60.00% <0.00%> (-21.11%)	⬇️
src/freqresp.jl	88.00% <0.00%> (-3.90%)	⬇️
src/utilities.jl	80.21% <0.00%> (-2.58%)	⬇️
src/types/Lti.jl	54.76% <0.00%> (-2.39%)	⬇️
src/types/SisoTfTypes/SisoRational.jl	72.58% <0.00%> (-1.62%)	⬇️
src/types/StateSpace.jl	89.28% <0.00%> (-1.20%)	⬇️
... and 1 more

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