[WIP] Plotting waveforms

[rossant]

• Started electrode module with a MEA class to represent a probe
• Waveform visual
• PanZoom utilities for VisPy
• Array utility functions

To do (possibly later):

• Better color management (store directly in HSV?)
• Use VisPy transforms for box placement
• Use ST instead of PanZoom
• Support sparse waveforms
• More interactivity options
• Better management of keyboard shortcuts
• Add depth

[coveralls]

Coverage decreased (-7.42%) to 90.39% when pulling c905bec on rossant:plot-waveforms into fefb263 on kwikteam:master.

[coveralls]

Coverage decreased (-6.99%) to 90.82% when pulling c9c5349 on rossant:plot-waveforms into fefb263 on kwikteam:master.