Add a convenience IO header

[MaelRL]

Summary of Changes

There is:

CGAL/boost/graph/IO/polygon_mesh_io.h
CGAL/PMP/IO/polygon_mesh_io.h
CGAL/IO/polygon_soup_io.h

The first one is a bit awkward to remember, so this PR adds a convenience header at CGAL/IO/polygon_mesh_io.h.

Release Management

• Affected package(s): BGL
• Issue(s) solved (if any): -
• Feature/Small Feature (if any):
• Link to compiled documentation (obligatory for small feature) wrong link name to be changed
• License and copyright ownership:

[lrineau]

CGAL/boost/graph/IO/polygon_mesh_io.h is still the documented header in the following doc pages:

BGL/group__PkgBGLIOFct.html
BGL/index.html
Polygon_mesh_processing/index.html
Shape_detection/index.html
Straight_skeleton_2/index.html
Surface_mesh_simplification/index.html

Dont' you want to change that as well, in addition to the changes in examples?

[MaelRL]

That was not my intention, but indeed that leaves an undocumented header in examples.

[sloriot]

Successfully tested in CGAL-6.0-Ic-129