Control Network Object should default to GeoPandas Object

We already have pandas and gdal as dependencies. We should just return the dataframe from an isis control network as a geopandas data frame. This avoids clients having to do stuff like:

```
gdf = gpd.GeoDataFrame(cnet, geometry=geopandas.points_from_xy(lons, lats))
```

This is a convenience item I believe, but will impact how we write out networks as well. We need to address #108 to confirm everything is working before adding this enhancement.

Start by reviewing issue #108 and the current ISIS control-network object and its dataframe conversion; the payload names no files or tests. Trace the existing network write-out path before deciding how a GeoPandas dataframe should flow through it. Done means the default returned object is GeoPandas-compatible and network writing still works.

Written by the indexing model from the issue text.