concat mask
lic date
nan fill
data_is_merged
np fill
Improved error message on unit conversion failure
Proof of concept for bounded forecast_period.
Add tests, fix bugs in proof-of-concept, and refactor.
Add integration test for format interoperability.
Update test results
Fix NAME-GRIB interop test
Fix doctests and extests
Merge pull request #1002 from bblay/conc-mask

concat mask
Fix GRIB save for bounded, non-hour forecast period.
Merge pull request #1019 from pelson/travis_fix

Pointed to direct link for Python NetCDF location.
Merge pull request #1014 from rhattersley/fpbounds-1.6.x

Reset PP/GRIB time points to middle of range.
Merge pull request #1011 from dkillick/conv_unit_fail_msg

Improved error message on unit conversion failure
Merge remote branch 'upstream/v1.6.x' into v1.6.x-conflicts

Conflicts:
	lib/iris/tests/integration/test_pp.py
	lib/iris/tests/unit/fileformats/pp_rules/test_convert.py
agg mask doc + tests
test ddof=0