Multiple phonon modes; frequency dependent complex impedance

A release with all of Brad's work as described in the paper: https://arxiv.org/abs/2203.16472

• Multiple phonon modes in the variational solution (~20% difference c.f. averaging)
• Multiple variational parameters (though we haven't yet found these useful)
• Multiple phonon modes in the transport theories (massive amounts of additional structure in temperature and frequency dependent mobility)
• Multicore evaluation & other efficiency improvements

What's Changed

• June 2021 grand merge by @jarvist in #14

Full Changelog: v1.3.0...v2.0.0