[GeoMechanicsApplication] Determine the adjacent continuum elements

[WPK4FEM]

The FindNeighbourElementsOfConditionsProcess contains much of this functionality, but is specific for conditions. Line conditions with opposite geometry direction are not found.

Communication about neighbouring elements to a geometry takes place through variable NEIGHBOUR_ELEMENTS. After looking for neighbours, all neighbours should have an entry in this list, such that later processes can rely on this during the current stage.

To get the functionality available in a reusable way, we propose to:
[ ] 1. Bring the geometry comparison to a separate class
[ ] 2. Add recognition of lines with reverse geometry in that class, with flexibility to either recognize or ignore reverse geometry.
[ ] 3. Use the new class inside FindNeighbourElementsOfConditionsProcess
[ ] 4. Write a class that recognizes which continuum element(s) is(are) adjacent to the 2 sides of an interface element.

Needless to write: all should be unit tested.