Meshing of 2DElements with internal openings only works if the external boundary is made of linear segments

#### Description:

Meshing of Panels with internal openings only works if the external boundary is made of linear segments.

It returns an **"element is self-intersecting"** error that seems to be due to a bug in Geometry_Engine.

This behaviour is inconsistent with the fact that, e.g. PlanarSurfaces are still meshed correctly even if their boundary is made of curved (arcs/circle) segments.

#### Steps to reproduce:

Mesh any 2DElement with external and internal boundaries where the external boundary contains some Arc segment.

#### Expected behaviour:

It should not throw the **"element is self-intersecting"** error and the meshing should work.

#### Test file(s):

See test files in #27.