Added image comparison to unittests for plots

- use image comparison from matplotlibs test framework
Changed plotting routines to use PropsSI method internally
Fixed python 3 bug in fromSI and toSI methods
Changed scale_plot method to use units argument

- use units argument for axis scaling of the plot (was unintenionally
  hardcoded to kSI)
- make method public to allow easy rescaling of plots
Merge pull request #183 from spinnau/plot_PropsSI

Changed plotting routines to use PropsSI internally