Add cooldown functionality

pyger/__init__.py:
`calc_constraints2` is added as an available function

pyger/base.py:
`find_long_hot_pitch_dwells()` and `calc_dwells2()` include the core cooldown code. Please see function docs for more details.

pyger/pyger.py:
`calc_constraints2()` is added to interface with `calc_dwells2()` in much the same way as `calc_constraints()` runs `calc_dwells1()`.