Demand table should allow for a user-defined future

[samgdotson]

Current behavior:

With DemandCommodity.add_demand(), users specify:

• initial demand (or "seed")
• growth rate
• growth method (linear, exponential, or logistic)

Desired behavior:

With DemandCommodity.add_demand(), users should ALSO be able to specify
the demand for each year rather than have the demand calculated automatically.
E.g.

init_demand=[100, 100, 100, 150, 150, 125]...

Rather than specifying a single number for the first year, an array/list of values were given.
These values would never be calculated by any method available, but represents a future where demand
suddenly skyrockets and then falls back down. Perhaps a future where everything is suddenly electrified followed
by significant development in efficiency in subsequent decades.

Guidance:

In commodity.py the DemandCommodity.add_demand() method should be updated with a conditional argument checking the type of the init_demand variable. If the type is a float or integer, the future should be calculated as it is now. If it's a list or an array, then the demand should simply be stored in the commodity demand dictionary. In the latter case, the length of the list should also be checked against the number of years specified by the user. They must match.