Minimize number of consecutive night shifts

The aim is to minimize the length of night shift phases, defined as consecutive night shifts occurring one after another.

Implemented using Google's OR Tools

src/cp/objectives/minimize_consecutive_night_shifts.py

penalties.append(
    sum(possible_night_shift_phase_variables) * (self._weight**phase_length)
)

For each non hidden employee we create variables that show potential night shift phases, meaning multiple night shifts after each other. The sum of those (meaning the number how many of those are assigned) is weighed. For longer phase length the weight is increased exponentially, penaltising long night shifts quite heavily.