Fix direct use of Unicode in PDController

[JacquesCarette]

https://github.com/JacquesCarette/Drasil/blob/10f9f9ed0e332c7a31875f6e035db96549c45945/code/drasil-example/pdcontroller/lib/Drasil/PDController/Unitals.hs#L15-L18

which was spotted while doing #2865 .

[balacij]

What symbols belong in the "Table of Symbols" of the SRS document? Is +/- infinite something we want to put in it?

Should all functions be displayed as the invLaplace shown there (e.g., with it's parameters)?

[smiths]

We wouldn't normally don't put mathematical symbols like +/- infinite in the table of symbols. We think of them as part of the mathematical background knowledge. In the terminology of theories that we discussed with @JacquesCarette, I think we can think of mathematical knowledge as a "context" theory. There has to be a point where we stop defining things, or we'll also be defining the symbols +, -, d/dt, etc.

For functions, I like when they are given with their type, but I don't think that is necessary in the table of symbols.

[balacij]

That makes more sense to me. Thank you! It's also interesting to think about how we could systematically retrieve information about the mathematical background knowledge. That seems it would need to be manually listed at the moment.

Does that mean we treat the "F(s)" component as part of L inverse's type (and that we should the parameter)?

[smiths]

We don't currently show the types of the other symbols, so no I don't think (at least for the moment) that we should show the parameter for L inverse. For gradient (∇) in SWHS and noPCM we don't show the arguments.

[balacij]

The syminvLaplace = variable "L⁻¹[F(s)]" is a bit more tricky. We'll figure this out soon (hopefully), when we re-work functions and vectors.

I'm not too sure what I was doing with my earlier PR, I think I was completely misunderstanding what @JacquesCarette said. I hope this PR is better!