Adding support for dates in perfect_foresight_setup

The user should be able to control the dates of the dseries produced by perfect_foresight_solver

• Currently, histval, histval_file and initval_file pass a dseries to perfect_foresight_setup
• By default, perfect_foresight_setup should be consistent with the dates of this dseries
• dates must be permitted in histval instead of only signed integers
• first_obs: sets the dates used for the first initial conditions. Must be consistent with histval or histval_file if they are used
• first_simulation_period: sets the dates used for the first period of the simulation. It is an alternative to specifying first_obs and must be consistent if first_obs is also used as an option. Values for initial conditions before this date must be available. Must be consistent with histval or histval_file if they are used.
• last_simulation_period: sets the date of the last period of simulation. It is an alternative to specifying periods and must be consistent if periods is also used as an option. Data for terminal conditions past the last period must be available