[bug correction] trirefine is now independant of triangulation numbering


*Correction in cubic interpolation function:*
the interpolated values on refined triangular grids should be the same regardless of the triangles ordering.

As of today, this was not the case for the cubic interpolation, as discovered by @ianthomas23 (refer to the new test enclosed in this PR, which was failing and should now pass). The differences were small but I consider this was a bug (at least highly unexpected).
The solution is a new choice for the Gauss quadrature points.

I added a new (non-image) test and updated the 2 baseline images involving this kind of cubic interpolation (one was failing as a result of this PR ; I also updated the 2nd to get a fresh start).

Note:
As changing the Gauss points was not my first guess (unfortunately...) I also made a double-check of the various shape functions used in ```_ReducedHCT_Element``` For this I found that it was more convenient to arrange them by columns, even if it this is not strictly pep8. If this is considered bad practise, please tell me ; I would be happy to revert.