Adds first revision of CubaIFGroup

A simple integrate-and-fire Neuron Group for use with current-based
synapses. cf. Vogels and Abbott 2005
Removes -Wall in standard makefile
Adds setter for all bg_currents in CubaIFGroup
Adds doxygen strings to STPConnection
Merge branch 'master' into Cuba
Adds example for STPConnection
Adds comment strings to sim_epsp_stp
Changes stimulation protocol
Finalizes STP example