another attempt at superfluid density/stiffness

[ffreyer]

Refs:

1. https://arxiv.org/abs/cond-mat/0303551
2. https://github.com/carstenbauer/dqmc
3. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.102.201112 or https://arxiv.org/abs/1912.08848
4. https://kups.ub.uni-koeln.de/7403/1/thesis_max_gerlach-online-final.pdf and its sources:
   • https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.68.2830
   • https://journals.aps.org/prb/abstract/10.1103/PhysRevB.47.7995

Checks:

• Visually checked EachLocalQuadBySyncedDistance for extended Honeycomb lattice
• Repeated Wicks expansion of current-current susceptibilities and checked against implementation

Changes:

• Fixed I - G case where I ignored the l = 0 case.
• Added notes on derivation as comments
• tweaked relative filepaths for BufferedConfigRecorder (unrelated)
• Adjusted longitudinal and transversal reciprocal vectors to what I believe they should be according to ref 4. I.e. longitudinal q's now point to the closest reciprocal lattice point that is parallel to the respective real space direction. Transversal are the same but perpendicular.

[ffreyer]

I'll do this in #137