convert E-I Net from MatLab to python

Convert the E-I Net neural simulator from MatLab to python (see http://www.pking.org/research/EINet/index.html for description). The simulator is 7,000 lines of MatLab code, see https://github.com/paulking/ei_net. The main simulation engine is in the file neurosim/NetModel_UpdateFast.m. neurosim/NetModel_Plot.m includes a lot of plotting code. neurosim/* is the simulator. The target language is python using numpy and Jupyter Notebook as an execution environment.