Add more router-based filtering

[rowleya]

In addition to the synapse filtering (#1633) it would be useful to see if we can do more filtering in the router. Previous attempts at fully copying the bitfield into the router were unsuccessful as it was impossible to compress the tables. A thought is that this was probably because the connections are random, and compressing random data is just not possible in general.

An idea is to group connections more and see if this leads to some reduction in the packets that finally reach the synapse filter before being filtered more. The idea would be to take the list of source neurons and split it into fixed-sized subgroups (probably of a power of 2 in length for simplicity in representation). Then, if any neurons in the subgroup are needed, add that to the table. If we do this in combination with the "simple" compression technique of merging adjacent entries when possible (as it already done in the routing table generator now), we may get a winning combination with the right division. At this point, any reduction in traffic would be good!