Area in 3D not computed in the correct way

[albiremo]

In 3D the result of the area are not computed in the correct way, hence we need:

• [x] a fix to the bug (probably looking at https://stackoverflow.com/questions/12642256/find-area-of-polygon-from-xyz-coordinates)
• [x] a unit test to avoid future mistakes

[albiremo]

Cleared in the merge https://github.com/albiremo/dualGPy/commit/9d66bf85ecb8f7ad04c3318f768b135ca5c7027e