* added helper methods us in As<Type> accessors that detect
  would be bad casts, where either there's no cast from one
  type to the other or the node is uninitialized. In these
  cases the cast would produce undefined behavior so an
  exception is thrown.
* fixed a bug in CurveAttributes ProcessOldVersions
  that the helpers detected.
* fixed issues in regression tests. updates to session files
  used in regression tests will be pushed in a follow up
  commit.


git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@23793 18c085ea-50e0-402c-830e-de6fd14e8384