Flow's full_load_time depends on time horizon

[p-snft]

Currently, we have parameters full_load_time_min and full_load_time_max, respectively.

    flow = solph.flows.Flow(nominal_value=2, full_load_time_min=4.5)

Assuming kW as the base unit for flows, means, that at least 9 kWh have to go through that flow. For short time horizons, that can be a low number, for time horizons lower than 4.5 h, the problem becomes infeasible. In my opinion, we should replace this by a minimum and maximum average load, respectively.

    flow = solph.flows.Flow(nominal_value=2, minimum_average_load=0.25)

This would mean 18/4 = 4.5 full load hours for a time horizon of 18 hours but 8760/4 = 2190 full load hours if a full year is optimised.