Fusion-solar-py

* Adapted `get_last_plant_data` to return `float` instead of strings

* Fixed bugs that prevented meter data to be correctly added to the plant stats
* Added additional combined statistics to the plant stats
* Updated the README to show new features

* Added a new function `get_station_list` that returns detailed info about all installed
stations under that account.
* Changed the functionality of `get_plant_ids` to only return the ids of stations. In some
setups, the ids returned here are ids of subunits (ie. dongle) that can't be used to extract
further information.

* Fixed a bug in `get_plant_stats`: Query time was by default set to the current time.
But the API expects this to be 00:00:00 of the current day.