Selection of mutiple weeks with the solve board

It is now possible to select multiple weeks with the solve board for solving the problem (with just one run of the solver). It is important for constraints or rules covering more than one week.

However when more than one week are selected, it is not possible anymore to use the stabilisation functionality. The stabilisation drop-down menu is automatically hidden when more than one week are selected.

One other thing to note is that the TTModel constructor's arguments changed. Instead of taking a weeks (int or list) and a year (int), it take a week_year_list object, which is a list of week_year dictionary with keys week and year. Sadly it was too complicated to solve the problem with weeks from different years, so if you try to solve such a problem, it will raise an exception.