Refactor FSSH main class and Wigner sampling functions

[felixmiaomiao]

For the main FSSH class:

1. Correct the calculation of the time-derivative coupling (TDC) for the κTDC method.
2. Correct the final half-step velocity update after a successful surface hop.
3. Improve screen output and trajectory writing.
4. Refactor the main FSSH class:

• Move independent numerical routines out of the class as pure functions.
• Encapsulate repeated or self-contained logic into helper functions to simplify the main kernel() routine.

For the Wigner sampling functions:

1. Add support for zero-temperature (T = 0 K) sampling.
2. Add input validation and sanity checks.