TriangleMesher: cell size to area conversion

[Huite]

Triangle requires an area for triangles, rather than a specified cell size. Gmsh uses a cell size.

To make things consistent across, I compute an area based on the cell size:

0.5 * cellsize * cellsize

I.e. $A = \frac{1}{2} b h$

This is assuming a right-angled triangled where base and height are equal. Of course, this means that the hypothenuse is longer than the specified cell size.

The ideal triangle is probably equilateral, in which case $A = \frac{\sqrt{3}}{4} s^2$.

This results in slightly smaller triangles.