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
pp soil level
Merge pull request #931 from bblay/ppsoil

Added PP soil level field code handling.
Merge pull request #985 from bblay/v1.6.0_branch

V1.6.0 to master
Added clarification notes
Improved wording of notes
Merge pull request #986 from dkillick/merge_docs_update

DOC: Clarification notes for (apparently) merge-related tools
Switch to biggus for deferred loading.
Fix PPField equality
Generalise iris.FUTURE for a set of options, and add 'lazy_results' option.
temp
newer ideas.TODO - collapse implementation.
Further tweaks.