Plot polar projection of ORCA grid

[jpolton]

Given a python script that does the job of nicely plotting properties over the North Pole:

1. Convert script(s) into a COAsT utility.
2. Add a demonstration notebook for plotting vectors over the north pole, using an example dataset
3. Add example dataset to ERDDAP server

Made a branch 0624/polar_quivers_notebook

[b-barton]

I think 1. of this is a continuation of issue #628

[b-barton]

I've made new notebooks for demonstrating the quivers (quiver_tutorial.ipynb) and a python example script for plotting the new dataset gc31 cropped to the Arctic (gc31_arctic_example_plot.py).

I've also added plotting functions for arctic plots:
coast._utils.plot_util.set_circle() which makes the axis shape round.
coast._utils.plot_util.plot_polar_contour() which reprojects data to enable plotting contours over the Arctic.

There is a new dataset at the link below which Jeff, Tobias and Thomas can access. The _so file has salinity which isn't used in the examples but may be nice for completeness/future examples.
https://nocacuk-my.sharepoint.com/:f:/g/personal/benbar_noc_ac_uk/ErXjdHUs2rVBn7ndaD6iwFsBZWCs60z-DOQZJZKFjCTrSg