Incorrect wall distance using axisymmetric mesh

On "axysymmetric" cases (where the mesh uses a radially extruded slice, with symmetry on the sides), the wall distance approximated by a diffusion equation (in ``distpr``) can be imprecise, with a difference up to 40% having been observed.

This issue also occurs using the CDO operators, so seems due to the the choice of equation or boundary conditions rather than linear operator.

In any case, this can induce an error on turbulence variables.

More works needs to be done on this, but replacing the approximate wall distance with a
geometrically computed wall distance for the boundary cells might at least mitigate the effects.

Attached is a small case allowing to test this.
[test_pipe_axi.tar.gz](https://github.com/code-saturne/code_saturne/files/3135105/test_pipe_axi.tar.gz)

从附加的 test_pipe_axi.tar.gz 用例和 distpr 的壁面距离计算开始；将其轴对称结果与 CDO 算子的行为进行比较。调查方程或边界条件是否导致了报告的 40% 差异，并评估针对边界单元提出的几何计算距离。完成的标准是该测试用例不再显示有问题的壁面距离或其湍流变量错误。

由索引模型根据 Issue 内容生成。