Downloading coincidence fails with to many stations

[153957]

As mentioned in HiSPARC/sapphire#30, when attempting to download coincidences for a cluster which has more than 32 stations the download will fail.
Perhaps we should change the download to work with subclusters instead of clusters.. but that is only a partial workaround..

Example:

Parameters
Cluster: Amsterdam
N: 6
Start: 2013/10/02
End: 2013/10/03

http://data.hisparc.nl/data/network/coincidences/?download=False&cluster=Amsterdam&n=6&end=2013-10-03+00%3A00%3A00&start=2013-10-02+00%3A00%3A00

[153957]

Partly fixed (9781965) by throwing error when a cluster is selected with to many stations. Not ideal, but at least tells the user why there are no coincidences returned..

[153957]

Untill this is fixed in SAPPHiRE/PyTables/NumPy I believe it is handled here correctly.
Once it is fixed and more arguments are allowed we can easily remove the checks..