Commit native fixedtotalnumber connector

Implemented the native version of the FixedTotalNumber connector.,It takes a fixed number of synapses and connects a source and target population with them.
In a distributed simulation, each MPI process first assumes that the target neurons are equally distributed among processes and then draws a random number of local synapses from a binomial distribution with varying parameter n and p.,
After that, each process draws local targets and global sources for its synapses.