Suggested updates to 2D plots etc

[pgleeson]

From changes in #213

• 2D plots should be -plotType Detailed by default, as they will generally not be too large, and it's more natural to want to see diams also.
• Why are "Detailed, Constant, or Schematic" with caps? Lowercase more pythonic?
• theme='dark' not supported in 2D plots. Not very likely to be used much, but the command line arguments suggest it is an option...

[sanjayankur31]

• the plottype option is shared between 2d and 3d plotter, so if I change the default for 2d, it'll also change for 3d. Leaving it as is for the moment.
• for the theme, I've improved the option doc to say that it is unused for 2d plotting at the moment.