Update definition of power entering and leaving the battery, update use of efficiency

[jakobgemassmer]

Implement a new definition of how to deal with efficiency:

• If we specify a charging curve (e.g. constant 100 kW), then this 100 kW comes from the grid. In other words, less power arrives in the (vehicle) battery due to the efficiency.
• When discharging a (vehicle) battery with 100 kW, 100 kW goes into the grid or wherever it goes. Meaning more power is taken from the (vehicle) battery due to its efficiency.

The (dis)charging curve therefore defines P_in and P_out in this picture: More info: https://smooth.readthedocs.io/en/latest/smooth.components.html#module-smooth.components.component_battery

Additionally: Ensure that the efficiency is considered in all strategies and for vehicles and stationary batteries! Example: In peak_load_window the stat. battery does not consider the efficiency when discharging.

[stefansc1]

This is already the behavior when using target_power of the battery load/unload functions. See test_components for example, test_charging and test_unload in particular. Do you have an example file where the efficiency is not accounted for, in your opinion?