Catch error if polygons are warped

[ole]

Petar Milevsky of Wollongong created a mesh with a warped polygon (see attached). A mesh was created, but excessive values for stage and momentum occurred. ANUGA should catch this condition and raise a meaningful exception.

See code in ​https://datamining.anu.edu.au/anuga/browser/anuga_work/debug

Turn it into a unit test and implement a proper exception.

[ole]

This is a polygon that causes a bad mesh

[ole]

A simple algorithm would check each segment against every other and use the newly developed intersection algorithm to detect crossings. The algorithmic complexity would be N^{2/2 if N is the number of segments, but given the expected size of polygons this seems like a good start.}

[James Hudson]

This error could probably be caught while building the octree.

[James Hudson]

An O(n log n) algorithm: ​http://www.lems.brown.edu/~wq/projects/cs252.html

[James Hudson]

NOTE: This is the same bug as #346. We need a fast algorithm to convert complex polygons to concave ones, or raise an exception on complex polygons with an algorithm such as this:

​http://www.lems.brown.edu/~wq/projects/cs252.html

I've already implemeted a brute-force check that has found some pathological data in the unit tests.

[James Hudson]

Rev 7690 - exception is raised on a pathological poly. An O(n log n) algorithm is used.

Also, one of the files used for the unit tests, mainland_only.csv, had a bad poly, and this has been fixed by hand.