deprecate cube iteration.
 : test included
revised cube iteration warning + test
 : clearer test code
 : don't use DeprecationWarning
Fixed issue introduced by #300.
Merge pull request #310 from pelson/fix_merge_bug

Fixed issue introduced by #300.
Tidy cube.__iter__ docstring.
Merge pull request #273 from pp-mo/no_cube_iter

control cube iteration
travis (full)
graphviz (for dot)