Allow using observable IDs in `observableFormula` and `noiseFormula`

[dweindl]

Following up on #543 and the discussion during the last PEtab editor meeting: There was general consent to allow using observableIDs in the noiseFormula column in the observables table.

What was not discussed explicitly:

• Should all observable IDs be allowed, or only the current one?
• If so, should it be allowed to use observable IDs inside observableFormula?

CC @PEtab-dev/petab-editors

Closes https://github.com/PEtab-dev/PEtab/issues/543

[fbergmann]

As to

What was not discussed explicitly:

• Should all observable IDs be allowed, or only the current one?

I'm fine with allowing all.

• If so, should it be allowed to use observable IDs inside observableFormula?

I don't see a problem with that, as long as this introduces no circularity, which you addressed in the PR.