Generalized longitudinal instability calculations

[murilobalves]

Include calculations of instability thresholds considering potential-well distortion effects.

Based on theory from:

• M. Venturini, "Passive higher-harmonic rf cavities with general settings and multibunch instabilities in electron storage rings" Phys. Rev. Accel. Beams 21, 114404 (2018)
• I. Karpov, T. Argyropoulos, and E. Shaposhnikova, "Thresholds for loss of Landau damping in longitudinal plane" Phys. Rev. Accel. Beams 24, 11002 (2021)

Temporarily implemented as methods of LongitudinalEquilibrium class. To be properly organized in some instability module In a future reorganization of pycolleff.

New important methods in LongitudinalEquilibrium class:

calc_synchrotron_frequency()
solve_longitudinal_motion()
calc_canonical_transformation()
calc_hmps()
solve_lebedev()
solve_lebedev_constant_frequency()
solve_oide_yokoya()