Triangulation on Sphere: Fix I/O

[afabri]

Summary of Changes

Add test case that illustrates Issue #8200.

Todo

• [x] Fix ToS2::is_valid() which does not compile (it is in operator>>(..))
• [x] Fix skip_vertex
• [x] Check why ghost faces are not the same after writing and reading back.

Release Management

• Affected package(s): Triangulation_on_sphere_2
• Issue(s) solved (if any): fix #8200
• License and copyright ownership: unchanged

[afabri]

@MaelRL can you have a look at the assertion. My guess is that the is_valid() function has a bug, as it did not even compile before I started this pull request.

[sloriot]

As you expect still some error in ToS tests of CGAL-6.0-Ic-245

   ERROR:      execution   of  issue_8200 in 0.0147595 s : Failed 
CGAL error: assertion violation!

[sloriot]

Successfully tested in CGAL-6.0-Ic-262