Issue/241 regular spiking source cell (#287)

Replace the 20 artificial spikes we inserted until now with spikes generated with a frequency spiking cell.
This is a cell that spikes regularly for a set period.
This connections outgoing from this cell mirror all the outgoing connections of cells with gid % 20 == 0.
This means that 5% extra connections have been added to the model to implement the equivalent behaviour with an 'implemented' neuron.

To implement this behavoir a new cell_group type has been introduced.
The fs_cell is extremely small and is not implemented on the backends.

Includes:
Unit test for the rss_cell functionality