Use non-deprecated methods to access OpenCascade Triangulation elements

[StefanBruens]

The Triangles() and Nodes() accessors have been deprecated in favor of the Triangle(index) and Node(index) methods, available since OCC 7.2.0, i.e. for over 5 years.

The Nodes() method has been removed with OCC 7.6.0, and Triangles() is also slated for removal (currently). Use the non-deprecated methods, and also fix the types of the index variables to Standard_Integer.

[StefanBruens]

OCCT commit adding the "new" methods: https://git.dev.opencascade.org/gitweb/?p=occt.git;a=commitdiff;h=624f732fb45531453af93b1806fd49fdcbbce633