Change default value for parameter next_planning_date

[MatteoLdx]

Default value for parameter next_planning_date (defined in beesdoo_shift) is set to 01/01/1970. When initiating a new database, this date must me changed manually to a closer date, because otherwise the first shift generation would be extremely long (due to the loop in method get_future_shifts).
Wouldn't it be a good idea to set the default value to a closer date ?