Add the option to define a time horizon for a demand commdity

[samgdotson]

Current behavior:

Demand commodities are defined with the DemandCommodity object and initialized for a new region with DemandCommodity.add_demand(). When users call .add_demand() they must supply

• region
• initial demand
• start year
• end year
• number of years

Also, if users set up a distribution for a particular demand, users need to call DemandCommodity.set_distribution() and further specify

• number of seasons
• number of hours

Desired behavior:
In order to simplify the input, users should be able to optionally initialize a commodity time horizon. This will be helpful when users add many regions. In other words, users should be able to call set_timehorizon() to specify this information once, since it will be the same across all commodities in a simulation.

# old behavior
DemandCommodity.add_demand(region='place', init_demand=500, start_year=start_year, end_year=end_year, n_years=N_years)

# requested behavior
DemandCommodity.set_timehorizon(start_year, end_year, n_years, n_seasons, n_hours)
DemandCommodity.add_demand(region='place', init_demand=500)

Guidance:

add_demand should check if the time horizon information has already been instantiated. If it has not been, and users have not specified it in add_demand an error should be thrown.

[nsryan2]

Is some of this issue (like the hours and seasons stuff) taken care of by the set_distribution function in the commodity script?