Added empty what's new 1.6
Merge pull request #778 from esc24/whatsnew16

Added empty what's new 1.6
Merge pull request #779 from rhattersley/abf-mask

Promote ABF/ABL and fix fill value and comparison.
Switch to biggus for deferred loading.
PPDataProxy exposes consistent dtype
Update tests.
Remove obsolete import.
Convert to Numpy/Ortho adapters
Update test
Revert test
Preserve old merge behaviour
Update test