To do file for Gaston

v 0.6:

* add demo for high-level functions
* improve high-level commands
* add support for map, filledcurves
* add support for colormap
* add support for parametric plots
* add support for set view
* add support for curve smoothing
* add support for setting ranges

v 0.7 and later:

* find a definitive way to fix races between Gaston and gnuplot
* decide if we'll support grid, ticks
* decide if we need to support more terminals
* review need to add a way to set screen terminal properties
* add debug options: print gnuplot command line, etc
* allow user setting gnuplot command
* look into the mmap interface to avoid creating files
* create script and data files besides plotting -- useful as a starting point
  to further refine the plot using gnuplot directly
* review the Dict type and see if some of our types should be Dicts
  (this would avoid the current 'empty fields' problem in some of our
  structures)

ongoing tasks:

* keep improving error checking (ie: don't give gnuplot invalid commands)
* keep improving test frameworks and the tests themselves
* keep improving documentation
* review type annotations to see if they can be improved
