P2P in batteries should become a participant

[ChaelKruip]

Part of https://github.com/quintel/etmodel/issues/1868

This is a battery with volume (kWh) and capacity (kW) depending on the number of units of the households_flexibility_p2p_electricity.

It stores excess according to the 'storage order' described in https://github.com/quintel/etmodel/issues/1965.

• When charging the efficiency is 0.85 (0.15 is lost) (see https://github.com/quintel/etdataset/issues/596)
• When discharging the efficiency is 0.89

The rules for behaviour are:

• If there is excess left in this time step (after possibly higher ranked technologies have been used), store as much as capacity and volume allow
• If there is demand (instead of excess) in this time step after volatiles and must-run have been treated, discharge to fulfil this demand as much as capacity and volume allow (after possibly higher ranked technologies have been used)

[jorisberkhout]

Changing milestone to 'Storage'.