PEP 8 iris._merge
Merge pull request #622 from rhattersley/pep8-iris._merge

PEP 8 iris._merge
Update PPA to get a working libgeos-dev
Merge pull request #634 from rhattersley/fix-ppa

Update PPA to get a working libgeos-dev
Convert iris.analysis.SUM to a weighted aggregator. Closes #595.