Add station_number property to Station class

A station_number can be given (optional) when adding a station to a
cluster.
If no number is given the station_id will be used for the
station_number.
Add a function to get a station based on station_number.
station_id is now 0-based, for consistency.
Improve variable name for API base url
Refactor ScienceParkCluster

Add warning when it can not connect to Public Database,
in which case it will use hard coded values.
Also add station 508 to the default set of stations.
Add two new classes for settings up clusters

HiSPARCStations for any combination of stations from the network.
The latest gps coordinates and the detector number is retrieved.
Detector positions are not yet taken into account (even for Science
Park).

HiSPARCNetwork creates a cluster objects for the entire HiSPARC Network.
Stations that have never uploaded a configuration will be skipped.
Add code example to main docstring of simulations.base
Use station number for column names (e.g. 's502')

s_index is still stored as a reference to the station in c_index.
The station groups are still ordered by station_id in s_index.