Add 'automagic' plotting interface module

[dopplershift]

Add a separate module for plotting that does more data management, provided the right data object/interface is being used:

• Use whatever data interface we define in data module
  • Variable naming, etc. done by data object
  • Units as done there
• Store data object, and then use this instead of passing data in
• Start with skewT
  • Keep name, just subclass from basic plotting module
  • Wrap methods from base class, just pass in data
  • default_lines() (?) method probably needs better name, but could add barbs and hodograph automatically
  • Add example

[dopplershift]

Leaving this to track adding a declarative version of SkewT.