Refactor new loads

[khelfen]

We decided to keep all loads in one DataFrame. At the moment charging points are handled separately from other loads. Heat Pumps need to be added as well.

• [x] Merge charging_points_df with loads_df in topology @khelfen
• [x] Merge charging_points_active_power with charging_points_active_power in timeseries @birgits
• [x] Merge charging_points_reactive_power with charging_points_reactive_power in timeseries @birgits
• [x] Change to_pypsa accordingly @birgits
• [x] Change #174 accordingly @khelfen
• [ ] Add heat pump functionalities @AnyaHe

[khelfen]

With #229 charging_points_df is now part of loads_df