Changed cube string repr

Changed cube string representation so that dim names are ```@``` if only aux coords are present and ```?``` if no coords are present, as opposed to the current system, which uses ```*ANONYMOUS*```.  In reaction to discussion here
https://groups.google.com/forum/?fromgroups#!topic/scitools-iris/i0-GHVNcpVc
and here
https://groups.google.com/forum/#!topic/scitools-iris-dev/DothjG_MUzA