[atlas] No more a list of probes when the measurement is ongoing?

That's probably a bug I created when I tweaked the API to handle 
explicit field requests properly.  I'll see what I can do to fix it 
today or tomorrow.

*However* if you're using `fields=` and are only making use of a few 
fields *you are inadvertently hammering our servers for no good 
reason*.  The `fields` modifier is there so you can request *specific 
fields*, not simply grab everything.  If you use it in this way, your 
request will work as you need:

    https://atlas.ripe.net/api/v1/measurement/1011817/?fields=probes

That'll return to you a list of all the probes used in measurement 
#1011817.  If you want the probes and the `msm_id`, then just do this:

    
https://atlas.ripe.net/api/v1/measurement/1011817/?fields=probes,msm_id

...repeat until you have *exactly* what you need and no more.  You 
limit your bandwith consumption and the load on our servers.  More 
importantly, the `fields=` trick is a hack that may not stick around 
for performance issues, so it's not a good idea to standardise on it.

With that said though, as it was working this way before I broke it, 
I'll do what I can to fix it now :-)