Plex: natural point SF

This branch modifies the notion of natural ordering and distribution. It corresponds now to the ordering and distribution implemented in the natural DM, i.e. the first DM with useNatural=PETSC_TRUE. The point SF sfMigration in DMPlexDistribute now maps the points from the current distributed DM to the natural DM. The natural SF sfNatural now maps the dofs from the current distributed DM to the natural DM. Corresponding examples ex15, ex26, ex26f90, ex62f90 in src/dm/impls/plex/tests/ are fixed. An additional example src/dm/impls/plex/tests/ex64.c checks the natural dof SF sfNatural. @blaisebourdin @knepley @sbalay: can you add labels and reviewers since this MR is from a fork. Thanks!