ENH: warning for not setting the crs of trackintel objects

[hongyeehh]

The current read functions in io would proceed without an error or warning when not providing a crs as input; the resulting trackintel class object thus has no crs set (e.g., running print(pfs.crs) will return None). This might cause problems later when running some analysis functions that require a crs, e.g., predict_transport_mode raises a warning: UserWarning: The CRS of your data is not defined..

Shall we add a warning statement if no crs is provided?