All choice boxes have deterministic defaults

Fixed a lingering issue caused by the use of hash maps that made choice
boxes randomly choose default values.

Now, tree maps produce the same defaults with certainty. These defaults
are as follows:

- all error size selections now default to "2σ"
- all expression type selections now default to "%"

In the future, we will want to futher refine these defaults by using
stored selections from the last session.

Additionally, I performed some small, internal refactorings on the files
that I was editing.
Merge branch 'dev' of https://github.com/CIRDLES/topsoil into dev
Rolled back to TestFX 3.1.0

Rolled back to TestFX 3.1.0 in order to fix ThreadDeath errors.