STDP implementation can be simpler

[jpalmer]

If anyone wants to do this, the STDP implementation can be simplified.

The idea is similar to what is described in Synaptic plasticity: taming the beast (review article in nature neuroscience). Also similar to the approach to splitting off the R/D variables

Basically, rather than using the lagstorage.c class, we store one new number per neuron.

On each timestep this is multiplied by a constant $C<1$. this depends on the tau in STDP.

On each spike, it is incremented by $A$ which is the same A as in the STDP window function.

Then when another neuron spikes, we just use this value to change the synaptic strengths.

This approach will remove a pretty significant part of the complexity of the calculation.

What can be easily removed/simplified if this is done:

src/lagstorage.*
stdp_change_calc (simplified)

DoSTDP also probably becomes simpler.

The other changes are that STDP_data uses the lags member and gets a different, simpler array instead.

Also, this will use less memory than before - (we go from a lagstorage/neuron to a float/neuron) so is faster because of that

also, the updates in lagstorage.c branch quite unpredictably so it should be faster to change to this new method