[atlas] filter ping results by AS

If I’m understanding your situation correctly, you want to see a list of 
measurements that include probes from a particular AS as their source. 
At present, there’s no easy way to make a single API call to get that, 
but you could synthesise that information by comparing two reasonably 
small bodies of data.

There are two bulk data downloads that you can use for this: measurement 
metadata <http://ftp.ripe.net/ripe/atlas/measurements/> (already 
mentioned, updated weekly) and the probe data 
<http://ftp.ripe.net/ripe/atlas/probes/archive/2015/10/> (daily).

Basically, you need to do three things:

  * *Compile a list of probe ids that represent probes within the AS you
    want*. You can do this by looping over the probes in the daily dump
    and capturing the id for any probe that has |asn_v4| or |asn_v6|
    equal to the AS in question.
  * *Compile a list of measurements using these probes*. You can do this
    by looping over the list of measurements, capturing the ids of
    measurements that have the aforementioned probe ids in the |probes|
    list.
  *

    Fetch the probe-specific data from the API using both ids:

    |https://atlas.ripe.net/api/v1/measurement/**measurement_id**/result/?prb_id=**probe_id**,**probe_id**&format=txt
    |

The above call will return all of the results for measurement 
|#${measurement_id}|, but only for probes with one of those two ids. 
That last bit, |format=txt| will ensure that the returning data is in 
bite-sized chunks so you don’t gobble up 30gb of memory just to parse 
what comes out.

Write that in a loop /with a reasonable wait time between each request 
or we’ll get mad at you/ and you’ve got a decent body of data to work with.

Note that there’s no reason to fetch the entire history of a measurement 
either. You can always pass |start=<timestamp>| to the above URL and 
it’ll only give you results returned since that date.

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