update to handle up to 3 dimensional cube so that we call scoreatprecentile with
only upto 2d data.
Merge pull request #625 from munslowa/percentile_agg

Percentile agg
Updated docs to use latest what's new.
added 42-byte wmo header, requiring code tidy
Merge pull request #610 from bblay/wmo42

added 42-byte wmo header, requiring code tidy
Merge pull request #676 from esc24/whatsnew1.5

Updated docs to use latest what's new.
Correct dim coord behaviour for aggregated by.
Merge pull request #681 from bjlittle/agg_fix

Correct dim coord behaviour for aggregated by.
fielfds file var res load
merge unique bug fix + comments
tests and pep8
pep8 test failure fix
pp_rules bmdi for bdx and bdy
minor pep8 fix
remove unique=False flag
arakawa staggering fix
added new stash cross refs
pep8
.get on stash to catch stash==0
review changes