[Athens, Greece] Add GTFS data for Athens public transport (https://data.hcap.gr/)

[saligari]

I've come across this site that seems to be up to date.

Here: https://catalog.hcapdata.gr/dataset/dromologia-osy is the bus & trolley bus information

and here: https://catalog.hcapdata.gr/dataset/dromologia-statheron-sygkinonion the metro/electric train as well as tram info.

I'm not familiar with GTFS or how this stuff works or I'd file a pull request. If someone is more knowledgeable or has the time, please give this a try.

Thanks!