Unhandled exception in TotalNetworkObjects

[jc-harrison]

Description
TotalNetworkObjects doesn't complain when initialised with level="lat-lon" and either network_object="versioned-cell" or network_object="versioned-site", but calling head() raises the following errors:

AttributeError: 'TotalNetworkObjects' object has no attribute '_df'

During handling of the above exception, another exception occurred:

ProgrammingError: column reference "lat" is ambiguous
LINE 5:                 (SELECT lat,lon, location_id,version,lon,lat...
                                ^

This only happens with level="lat-lon", and is not a problem when network_object="cell".

To Reproduce

tno = TotalNetworkObjects(start='2016-01-01', stop='2016-01-07', network_object="versioned-cell", level="lat-lon")
tno.head()

Expected behavior
I'm not sure what the correct behaviour should be, but I guess this should either work or an exception should be raised by __init__().

[greenape]

nice.. duplicate lat & Lon column and ambiguity because versioned-x carries a lat-lon. Either needs a special case to drop those cols from the versioned-x inner query, or to not accept lat-lon. Think former is better, because this query essentially lets you ask what cells are alive per tower?