Cubic interpolation for triangular grids

This is the second and final PR for issue #1521.

It adds a `CubicTriInterpolator` to interpolate scalar fields defined on triangular grids, and a utility class `UniformTriRefiner` to perform high-resolution tricontouring by grid refinement.
A utility class `TriAnalyzer` is also added, which performs basic triangular mesh analysis.

I have included tests for the 3 classes, 3 pylab_examples which describe the functionalities, and have updated sphinx docs, whats_new and CHANGELOG.

Regarding the documentation, I did my best to follow MEP10 ; any comment from MEP10 specialists is highly appreciated.
(I make here the observation that, as of Numpydoc 0.4, a `__call__` method is not exposed in the documentation unless explicitly listed. Also refer to https://github.com/numpy/numpy/commit/7a57dc108fe8ee981439aea38df0d35cfbffaf4c).

This is my first matplotlib submission; my warmest thanks goes to @ianthomas23 for his very detailed instructions throughout the process. Any remaining mistake is mine.