[Kratos] issue with geometry volumes and areas

[Jordi]

Dear Kratos users and developers,

we have recently detected that some geometries (in particular 
tetrahedra_3d_4.h) always return positive values in calls to 
Geometry::Area() and Geometry::Volume(). This could be an issue, since 
these functions are being used in Element::Check() to detect inverted 
elements, which should have negative size. We are considering removing 
the absolute values and allowing areas and volumes to be negative, which 
would enable the use of these functions to detect incorrect input.

The question here is: do any of you rely on these values being positive 
in your applications, even when local numbering is wrong? If we removed 
the absolute values and allowed for negative returns, would this cause 
your code to fail?

If no one has a problem, we will go ahead with this change in the 
following weeks.

Greetings,

Jordi