Show when no masked elements in CML.

In cases where Cube.data is a masked array, but where the mask is all False (i.e. no elements have been masked out) this converts the CML representation from an opaque checksum (which is purely dependent on the size of the array) to the string "no-masked-elements".

This has some minor benefit in understanding the existing test results, but is also expected to have a benefit if we switch Cube.data to *always* be a masked array (as discussed in #900).