Lagrange Release v6.1.0

Infrastructure

• Added documentation for open source repo.
• Minor cmake and documentation cleanup.

Core module

• Update to polygonal mesh API:
  • Added SurfaceMesh::get_attribute_name() .
  • Released requirement for Attribute::get()
• Added support to triangulate a polygonal mesh.
• Cleaned up lagrange::range() and dropped support for range_vertices(), range_facets() and row_range().

IO module

• Added MSH format support via MshIO.
• Clean up polygonal IO.