Control Network Object should default to GeoPandas Object

[jlaura]

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.