Added soliscloud_station_index configuration item

Till now, always the first station in SolisCloud was retrieved.
But it is possible the first station is not the station you want to retrieve data from, see this issue: #29

The following configuration item has been added to soliscloud_to_pvoutput.cfg:

soliscloud_station_index = 0

The soliscloud_station_index information has been added to the README:

The soliscloud_to_pvoutput.py script will get the station id via the configured soliscloud_station_index (default the first station) with the secrets of SolisCloud (see next section). Thereafter it will get the inverter id and serial number via the configured soliscloud_inverter_index (default the first inverter).

And this section has been updated: Configuration with multiple inverters in one SolisCloud station