[FEATURE] Improve connection of charging points

[birgits]

• Connect home charging points to residential buildings and work charging points to CTS and industrial loads when connected in voltage level 7
  • Here, the potential LV buses should in case of home charging only be residential loads and in case of work charging industrial or CTS loads
  • This is e.g. already done in function Topology.connect_to_lv()
  • In case no suitable load is available, the current approach can be used, that the charging station is connected somewhere in the grid
  • In case a suitable load can be found, the charging point should be connected directly to the same bus as the load
  • A load is only suitable, if number of connected charging points is below defined value as done here
  • Connecting to a suitable load bus could also be made an option when calling the function
  • Same can be done for heat pumps, storage units and generators
  • Heat pumps with sector 'individual_heating' or 'individual_heating_resistive_heater' should be connected to some load bus
  • Solar generators in those voltage levels should be connected to some load bus
  • Storage should be connected to bus with a solar generator
  • Same could be done for voltage level 6
  • Same could also be done in MV, but here, the distance from the geolocation of the component to the selected bus should not be too large; in case it is too large, the component should just be connected based on its geolaction
• Charging points public - if next grid connection point in LV is too far away, check whether next grid connection option in MV is much closer
  • Needs to be implemented here
• Consider distance from grid in attractivity of public charging points - @khelfen could you please add here where best to do it? I think here, but I don't fully understand this function. Maybe elaborating the docstring would help.

[khelfen]

I have updated the docstring and comments within the mentioned function get_weights_df a bit. You can find it in PR #416. At the moment the tests fail, but I think this is due to your changes @joda9 ?

The function already includes the distance to the next MV/LV substation:

https://github.com/openego/eDisGo/blob/d68e4d4c609bcc36f0fbd4ed49526107f527a6e7/edisgo/io/electromobility_import.py#L725-L727

This is based on the distance calculation here:

https://github.com/openego/eDisGo/blob/d68e4d4c609bcc36f0fbd4ed49526107f527a6e7/edisgo/network/components.py#L893-L918

At the beginning of the process of assigning charging processes to charging points, it is not clear if a charging park (charging park consists of one or more charging points) will be connected to LV, MV/LV, MV or even HV/MV level. Also, when the code was written, the LV level wasn't geolocated yet. So I used the distance to the next MV/LV substation as a guess.