Merge branch 'featureMeshConversion' into 'master'

Feature mesh conversion

implements a cast from polyMesh to Mesh and vice versa using  static_cast<Mesh>(polymeshInstance) or static_cast<PolyMesh>(trimeshInstance)
as mentioned in #23 .

Copy constructors for respective mesh types were added as well as a function to copy all properties from one mesh to another.
Unittest for mesh conversions were added checking the number of vertices / faces as well as custom double vertex properties and edge properties.

Note: The used triangulation only works for convex polygons, it should be replaced by the earclipping triangulation mentioned in #33

See merge request !110
23 jobs for master in 40 minutes and 56 seconds (queued for 3 seconds)
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