Allow user-defined weight scale / left shift value in ring buffer

[andrewgait]

In synaptic_manager.py, the code in _get_ring_buffer_to_input_left_shifts does a calculation to scale the ring buffer and currently attempts to balance accuracy against too much overflow. There is a user request to be able to bypass this calculation step and set a user-defined value which controls the scaling / left shift from a higher level - either from the model or even from the level of a PyNN script.

[chanokin]

I think it's related to my question/issue Some connectivity patterns end up eliminating STDP parameters due to scaling.

[rowleya]

Yes, this is directly related. Both solutions should help in different ways; this one is about control, the stochastic rounding will help on longer running simulations regardless of the minimum representable value.

[andrewgait]

Noting here that this should probably be dealt with at some point, but it doesn’t need to happen now.