Minor update to top-level readme
Merge pull request #978 from esc24/readme_update

Minor update to top-level readme
aggregate with string coords

test
Merge pull request #980 from marqh/flex_agg_sq

aggregate with string coords
DOC: Updated whatsnew to 1.6.0
broken links (install)
Merge pull request #982 from marqh/brokenLinks

Broken links
Merge pull request #981 from cpelley/up2date_whatsnew

DOC: Updated whatsnew to 1.6.0
Switch to biggus for deferred loading.
Fix PPField equality
Merge pull request #985 from bblay/v1.6.0_branch

V1.6.0 to master
Added clarification notes
Improved wording of notes
Docstring for Cube.lazy_data()
Merge pull request #986 from dkillick/merge_docs_update

DOC: Clarification notes for (apparently) merge-related tools
Convert Cube.has_data() to Cube.has_lazy_data()
Update travis to biggus v0.3
Merge pull request #900 from rhattersley/biggus

biggus
WIP
CF aligned alternative implementation of hybrid pressure