Improve the example scripts #245

The current set of example scripts covers a rather limited range of functionality, with a lot of very similar examples, many of which were written a long time ago.

We should retire some of the old examples, and write some new ones, showcasing a wider range of PyNN's current functionality. The examples should also generate (optional) plots, where appropriate.

Ideas

• an IF neuron and an HH neuron with different levels of current injection
• single IF neuron receiving synaptic input from a Poisson source
• population of Izhikevich neurons, each with a different parameterization, responses to current injection
• ditto for AdExp neurons
• a 2D population with spatially-modulated input currents
• a demonstration of synaptic facilitation and depression
• a comparison of different STDP weight-dependencies
• a demonstration of different current sources
• Brunel
• Vogels-Abbott CUBA/COBA benchmarks
• a native NEST example
• a native NEURON example
• a demonstration of distance-dependent connectivity
• an example of writing a new connector, e.g. giving Gabor-shaped connectivity.

Please feel free to add ideas below...

Checklist for examples

• well commented
• complete docstring, include usage instructions
• has "--plot-figure" command-line option