Geogram-like LabelingGraph interface

[sebmestrallet]

I like the GEO::Mesh interface:

mesh.vertices.nb();
mesh.facets.corner(facet_index,local_vertex);
mesh.facets.create_triangle(v0,v1,v2);
mesh.cells.are_simplices();

For the include/labeling_graph.h LabelingGraph struct, instead of:

lg.nb_charts();
lg.nb_invalid_charts();
lg.facet2chart[facet_index];
lg.chart[lg.boundaries[chart_index].left_chart].label;

we could have:

lg.charts.nb();
lg.charts.invalid.nb();
lg.charts.index_from_facet(facet_index);
lg.boundaries.left_chart(boundary_index).label