Use the QC partition function by default in Torsion

[alongd]

Thanks @mjohnson541 for spotting this...!
This PR changes the default of Torsion (practically of HinderedRotor) to quantum=True.
From playing around with and without this flag on, I got only insignificant changes to thermo (e.g., less than 0.1 cal /mol*K). Speed wasn't affected either.

Also, a bunch of minor changes to RMG's statmech were added to this PR.

[mjohnson541]

So I've been planning to add Q2DTor and classical rotors along with the QM change in one PR, which will have nasty merge conflicts with this if you're making white space/stylistic changes. Can this wait until after that?

[alongd]

When will Q2DTor and ND rotors be PR'd?
I can remove the white space stuff, maybe let's get this PR in for the new release soon?

[mjohnson541]

Q2DTor is already PR'd...ND classical rotors is finished...I'm just tweaking/testing rotor frequency estimation for the semi-classical correction. Should be ready soon.

[mliu49]

Could you decide whether this, #1585, and classical rotors are intended to be included in the 2.4.0 release?

[alongd]

This PR is not intended for the next release.
I'll modify it soon

[alongd]

I'm migrating some of the commits here to a different PR (which will discuss a different topic), closing this one in favour of other PRs with higher precedence.