The ext_neuron class used for two distinct functions slows down execution

[golosio]

The ext_neuron class is used both to represent the images of NEST (CPU) neurons, through a software interface, and the images remote source neurons communicating spikes to their images through MPI. In principle this could be acceptable, and does not yield errors, however the call to the kernel Update functions of this model, which is necessary for the NEST interface, is unnecessary and slows down execution in case of the image of remote source neurons. For this reason, for now it would be better to have two separate classes (models) for the two cases.