Make some adjustments of weight / STDP values to make examples better

[andrewgait]

Some adjustments of weight / STDP values were needed to make the examples behave as they had done previously following the merge of the neuromodulated STDP in master and the weight_scale branches. These are technically separate changes from that branch in that I don't think they affect master so much, but they are required if the weight_scale branches are merged.

"Depends" on https://github.com/SpiNNakerManchester/sPyNNaker/pull/852