Part 1 of plotting fixes

This fixes problems 1 and 2 outlined in issue #3 and replaces the fixes in merge request !25.

The pull request adds:

• A (mostly complete) docstring to the plot function
• Adds some more useful error messages
• Reorders some of the logic to:
  • only give a Dgrid error if we actually need the Dgrid (this isn't always the case.)
  • checks if we need fpath_ckdtree. If we need it and the file isn't found we get useful suggestions as to why this might be.
  • raise errors if incompatible grid_type and plot_methods are called.

The pull request is passing tests and work on the code from issue #3..