Changes in Version 0.1-2
  
  o  Added option to crop spatial domain using polygons.

Changes in Version 0.1-1
  
  o  Sorted z levels to make color specification much easier when plotting.
  
  o  Added option to limit raster extent in the four coordinate directions.
  
  o  Fixed error with list construction; error sometimes occurs when specifying
     breakpoints.
  
  o  Updated examples in documentation