trac #15623: Immutable graph backend for Posets
Trac 15623: Let to_graph return an immutable graph
trac #15623: rebase on 6.1.beta4
trac #15623: Rebase on updated #15622
trac #15623: Removing the hack from #15622, update a variable's name
Fixed complement for immutable graphs.
Fixed doctest output.
trac #15681: Name of immutable graphs
Merge branch 'u/ncohen/15681' into public/graphs/complement-15699

* u/ncohen/15681:
  trac #15681: Name of immutable graphs
  trac #15623: Removing the hack from #15622, update a variable's name
  trac #15622: bugfix before #15623 gets merged
  Trac 15623: Let to_graph return an immutable graph
  Trac 15622: Review commit, fixing a misspelled doctest
  trac #15622: More informative exception message
  trac #15623: Immutable graph backend for Posets
  trac #15622: Immutable graphs must not be relabeled
Made complement of an immutable graph be immutable. Fixed doctest due to merging #15681.
trac #15681: A doctest for the bugfix
trac #15681: Rebase on6.1.beta5
trac #15681: New doctest for name() on immutable graphs
Merge branch 'public/graphs/complement-15699' of trac.sagemath.org:sage into public/graphs/complement-15669

* 'public/graphs/complement-15699' of trac.sagemath.org:sage:
  Made complement of an immutable graph be immutable. Fixed doctest due to merging #15681.
  trac #15681: Name of immutable graphs
  Fixed doctest output.
  Fixed complement for immutable graphs.
  trac #15623: Removing the hack from #15622, update a variable's name
  Trac 15623: Let to_graph return an immutable graph
  trac #15623: Immutable graph backend for Posets
trac #15699: fix hasattr in generic_graph.py.
Merge branch 'u/ncohen/15681' of trac.sagemath.org:sage into public/graphs/complement-15669

* 'u/ncohen/15681' of trac.sagemath.org:sage:
  trac #15681: New doctest for name() on immutable graphs
  trac #15681: A doctest for the bugfix
Changed hasattr to getattr.
Merge branch 'develop' into ticket/15623

* develop: (223 commits)
  Updated Sage version to 6.1.rc0
  only use CONFVERSION after it has been incremented
  note that the E6->A5 rule is now implemented
  document the glibc bug workaround patch
  Avoid scanf instead of just ignoring rest of file
  bump confball version
  fix for the m4_divert_push error
  sage-clone-source: allow downloading the configure tarball
  Require autoconf 2.64
  revised thematic tutorial regarding maximal_subgroup(s) method
  reference: branching_rules: minor fixup
  bump confball version
  removed bogus commits
  LinearMatroid.is_field_equivalent now tests if self is other before calling LinearMatroid._is_field_isomorphism.
  implemented maximal_subgroup(s) as a WeylCharacterRing method etc.
  Updated Sage version to 6.1.beta6
  Do not use the GNU extension chmod --recursive
  Workaround for glibc-2.18 scanf bug when building ATLAS
  Added  free_group and finitely_presented_group
  Made the repr sorted for the rules.
  ...

Conflicts:
	src/sage/graphs/base/static_sparse_backend.pyx
Re-insert a bit of code that had been erroneously deleted in the previous merge commit
Merge branch 'u/ncohen/15681' of trac.sagemath.org:sage into u/tscrim/15681

* 'u/ncohen/15681' of trac.sagemath.org:sage:
  trac #15681: New doctest for name() on immutable graphs
  trac #15681: A doctest for the bugfix
  trac #15681: Name of immutable graphs