Seperate discharge and charge for BEV

[Pyosch]

In the charge function of the VPPBEV class the discharging of the car while not at home and the charging of the car while at home are combined.
Seperate these two and enable charging strategies.
Maybe combine with a usecase/strategy

[Pyosch]

In addition we need to add a state_of_charge attribute to the bev class