More options for 'plot_length_units' and results caching

[yuanshen-flexcompute]

This is hopefully a simple request to add more options to plot_length_units for simulation:

• mil (1 mil ~= 25.4 microns)
• inch (1 inch = 1000 mils)
  These tend to be common units used in RF.

Also, I noticed that changing plot_length_units will cause the simulation to re-run completely even when there should be a cached result. Since this option is cosmetic and shouldn't affect the simulation outcome, is it possible to leave it out of the caching check?

Thanks.