Mesh_3 deprecated mesh domains removed in CGAL 6.0 Beta 1

[joachim-reichel]

Compiling pygalmesh from Debian unstable against CGAL 6.0 Beta 1 fails since <CGAL/Implicit_mesh_domain_3.h> is missing. Please check whether it is possible to include a legacy header including the replacement headers with a suitable deprecation message. Otherwise, it might be a good idea to describe in the release notes how to modify the affected code.

Compiling octave-iso2mesh from Debian unstable against CGAL 6.0 Beta 1 fails since <CGAL/Labeled_image_mesh_domain_3.h> is missing. Please check whether it is possible to include a legacy header including the replacement headers with a suitable deprecation message. Otherwise, it might be a good idea to describe in the release notes how to modify the affected code.