Locate max cores of a chip

[alan-stokes]

this support the usage of sdram links etc much better, as we can now partition to a machine setup. Does assume that the spalloc max machine data is updated to include black listing

[coveralls]

Coverage increased (+0.03%) to 64.525% when pulling 6df6f7e on locate_max_cores_of_a_chip into a03dc61 on master.

[rowleya]

This looks like it should work. My only comment is that this could also have been done like the SDRAM tracker with a ValueOrderedDict of _chip_n_cores[x, y]. This might be required in future if this seems to be slow when the number of cores increases.