Separate data/mask in PP repr

Makes the repr of PP fields with masked data consistent between NumPy 1.6 and 1.7.

It does this by avoiding the MaskedArray repr and explicitly outputting the `data` and `mask` components separately.

NB. No extra tests because the existing ones already fail on 1.7 without this patch.

Fixes:
 - iris.tests.test_pp_module.TestPackedPP
  - test_rle