BEP-19: Plotting

Abstract: It would be nice to have a few functions to create publication-ready figures.
It seems to me that we currently duplicate a lot of code every time we need to create
our figures. Some of this is Brian-specific, but most is probably not.

Style options
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* colors (BW default)
* figure size: possibly choices for different publications (with choice of 1 or 2 columns)
* fonts: could also be defined from publication styles
* point size (line thickness) and symbol size (cross, dot, etc)

General plotting functions
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* add letters to panels (A, B, C...)
* axis scales with horizontal/vertical segments
* insets (e.g. zooms)
* ticks (not too many etc)

Brian-specific plotting functions
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* Add spikes to traces (uses a spike list and a state monitor)
* Remove reset lines from traces
