Postgis should warn when no index column is provided

[henrymartin1]

The index of a dataframe plays a very important role in trackintel. Therefore, we included a warning if no explicit index column is provided and the default index is assumed when csv files are read e.g., here I think it would be very helpful to have the same functionality for the read postgis functions (forgetting to set the index column actually caused errors for me several times...)