add descriptions to variables and parameters

[baggepinnen]

This PR adds [description = "sting"] to a number of variables and parameters in the Blocks and Rotational modules. Other modules will come in separate PRs.

This serves two purposes

1. Make models from model libraries easier to work with. Without a description, it's hard to know what a variable called k deep inside a model does, in particular if the model comes from a library.
2. Illustrate best practices. People are likely to look to MTKstdlib to learn how to build component libraries, and I hope to establish adding variable descriptions as a best practice people follow.

[codecov]

Codecov Report

Merging #135 (4c7b3ef) into main (a821c7a) will increase coverage by 0.67%.
The diff coverage is 97.91%.

@@            Coverage Diff             @@
##             main     #135      +/-   ##
==========================================
+ Coverage   65.34%   66.01%   +0.67%     
==========================================
  Files          33       33              
  Lines        1434     1436       +2     
==========================================
+ Hits          937      948      +11     
+ Misses        497      488       -9     

Impacted Files	Coverage Δ
src/Blocks/utils.jl	68.00% <85.71%> (+6.46%)	⬆️
src/Blocks/continuous.jl	98.20% <100.00%> (+0.03%)	⬆️
src/Blocks/math.jl	100.00% <100.00%> (+3.79%)	⬆️
src/Blocks/nonlinear.jl	100.00% <100.00%> (ø)
src/Blocks/sources.jl	92.45% <100.00%> (ø)
src/Mechanical/Rotational/components.jl	100.00% <100.00%> (+4.34%)	⬆️
src/Mechanical/Rotational/utils.jl	93.02% <100.00%> (+5.21%)	⬆️

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[baggepinnen]

JuliaFormatter does not appear to understand the @variables macro, when formatting the continuous.jl file, it tries to change the syntax

xd(t)=xd_start [
        description = "Derivative state of SecondOrder filter $name",
    ]

from short to long function definition:

function xd(t)
        xd_start
    end [
        description = "Derivative state of SecondOrder filter $name",
    ]

domluna/JuliaFormatter.jl#669