Extend model with battery

[LMerbecks]

This has really low priority we want to do everything else first.

Add a battery to the model that can be loaded by the pv system and discharges to make up for excess load.

We could add two decision parameters to the model then. One for the maximum load supported by the battery and one for the capacity of the battery.

We really need to constraint these decision variables though as it is to be expected that the capacity and load supported will rise to the maximal values asap.

[LMerbecks]

okay let's see what we want for this: we want a battery that can store some amount of charge in kWh and can discharge a maximum power measured in kW

I think we can model this relatively easy after the fact as we already have the power and demand curves. we can then iterate through these arrays and fill the battery or drain it when it is needed. For simplicity let's start with an empty battery.

Also let's keep the battery state and power flow in an array so we can do nice plots afterwards.

[LMerbecks]

done with #15