Fix bugs, add `calc_dwell2_stats()` function.

- pyger/__init__.py
  - Add `calc_dwell2_stats()` to available function list.

- pyger/__pyger__.py
  - Add definition of `calc_dwell2_stats()`.
  - Add `**stateskw` keyword argument input to all `_get_states1()` functions for all model class definitions. This allows one to specify a group of initial states into the cooldown constraint calculation routine, regardless of whether or not each state is used by the model class. Otherwise, `calc_constraints2()` would need to be run for each model separately. Without this fix, a TypeError exception is raised if one specifies a keyword, such as `n_ccd` for a group of models where one of them does not use this keyword.