ENH: Enable spatial_filter for all trackintel GeoDataFrame classes #542

Currently Triplegs, Staypoints, Locations have the spatial_filter method.
But this should also be enabled on Positionfixes and TripsGeoDataFrame.
As there are no further requirements on the GeoDataFrames we can easily implement it in TrackintelGeoDataFrame and all subclasses will inherit it. Additionally @doc can be used to update the docstring of TrackintelGeoDataFrame.spatial_filter accordingly.