Convert pid parameters to :parallel

[mzaffalon]

Using standard fails when KP=0.

[mzaffalon]

Addresses #897.

[codecov[bot]]

Codecov Report

Attention: 57 lines in your changes are missing coverage. Please review.

Comparison is base (6e6a0d1) 34.06% compared to head (0ac6a75) 34.03%. Report is 3 commits behind head on master.

Files	Patch %	Lines
lib/ControlSystemsBase/src/pid_design.jl	0.00%	57 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #900 +/- ## ========================================== - Coverage 34.06% 34.03% -0.03% ========================================== Files 42 42 Lines 4624 4642 +18 ========================================== + Hits 1575 1580 +5 - Misses 3049 3062 +13 ```

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[mzaffalon]

Two questions:

1. how can I increase code coverage?
2. what is the purpose of @. in, say, https://github.com/JuliaControl/ControlSystems.jl/blob/master/lib/ControlSystemsBase/src/pid_design.jl#L559

[baggepinnen]

1. By adding tests that causes the added lines of code to be executed.
2. 

[mzaffalon]

2. ![image](https://user-images.githubusercontent.com/3797491/282374098-731b5403-32d5-4968-8edb-998e8ec99470.png)

Thank you. So it has no use on this specific line?

[baggepinnen]

It did have a use, it supported parameter arrays as inputs

julia> ControlSystemsBase.convert_pidparams_from_standard(rand(3), rand(3), rand(3), :parallel)
([0.17773398598165757, 0.32161078829707446, 0.722649466067516], [0.2515777728255106, 0.36780970052897355, 0.7964910544026514], [0.09807961789854291, 0.2953669775189507, 0.042697386412817936])

with the refactoring currently applied in this PR, that functionality was broken. One could argue whether that functionality was needed or not, but since it was there we shouldn't break it

[mzaffalon]

Thank you. I hope I didn't forget any.

[baggepinnen]

Very nice, thanks for your hard work :)

[mzaffalon]

My pleasure and thank you for your patience.