Add getter for observed variables

[hersle]

Suggestion based on discussion in #3555 to "prevent" stuff like map(eq -> eq.lhs, observed(sys)) for a little nicer API.

My idea was to introduce a convenient observeds getter named symmetrically to unknowns.
But working through it, I realize that this leads to an "asymmetry" in the naming of equations and observed(s).

If you think this is a helpful addition, what is the best name?

• observeds(sys)?
• observed_variables(sys)?
• unknowns(sys; observed = true)?
• something else?

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

[TorkelE]

I think observeds soudns quite weird, also, it doesn't directly distinguish from observed. observed_variables sounds better, but something shorter would be nice. Maybe either observed_vars, observed_syms would be good?

Alternatively, observables might work?

[ChrisRackauckas]

I like observables

[ChrisRackauckas]

It captures that these are lazily computed on-demand only if asked for, which is a use in computer science and matches this quite well. But then it also is a physical observable, can be measured from the system. It's a good fit.

[hersle]

Thanks, I think observables sounds great! Will change it when I get back.