Core::TriangleMesh naming consistency

[dlyr]

Core::TriangleMesh have vertices and normals method, they should be renamed to getVertices and getNormals as the Radium-Engine coding style require.
More over, we need to remove the special case of normals, even if we can have an alias as getNormals (in place of getAttrib(getAttribHandle<Normal>("in_normals"))).getDataWithLock()), but regarding recent changes, we should have a haveNormalsorisNormalsAvailable` to state if the normals are really present in the mesh.