implemented transformation of parameters for random distributions specif...

...ying synaptic weights.

NEST describes synaptic currents in pA while pyNN uses nA.
When random distributions for weights are chosen, the pyNN.nest backend now forwards the drawing of the random number to NEST with its internally implemented distributions. Therefore, the parameters of the given distribution have to be tranformed so that the distribution is correctly transformed.

This branch implements the parameter transformations for which the transformation is possible. It is not possible for binomial and poisson distribution for mathematical reasons.