Correct input handling for gif_cond_exp_multisynapse

[heplesser]

This issues becomes relevant once #261 is merged.

Input handling for gif_cond_exp_multisynapse needs to be modified according to the decision made in the NEST Developer VC 19 Sep 2016:

• Replace E_ex, E_in with vector of reversal potentials the same size as taus_syn (one per synapse)
• Replace I_syn_ex, I_syn_in recordables with one recordable I_syn representing the total input current
• Update class and handle(SpikeEvent&) to a single input buffer per rport, see aeif_cond_beta_multisynapse for an example; consider adding a check for non-negative weight.
• Update *_dynamics function to handle input current correctly, along the lines of

  double I_syn = 0.0;
  for ( size_t i = 0; i < node.P_.num_of_receptors_ ; ++i )
  {
      I_syn += y[ S::G + i ] * ( node.P_.E_rev[i] - V );
  }

• Move *_dynamics function to cpp-file.
• Provide examples (python based).

See also #509, #510.