Make MEAN with weights lazy

[bouweandela]

Closes #3129

Note that this does require at least version 1.1.0 of dask, I'm not sure if that is something you want.

[bouweandela]

I have just signed the CLA.

[valeriupredoi]

there should be no conflicts on version since I have been using dask=1.1.1 for a while now and conda is not complaining wrt iris-dask interdependency; pinning is needed for the func to work tho as @bouweandela says

[valeriupredoi]

also linking the original ESMValTool issue so we can close that when this gets merged https://github.com/ESMValGroup/ESMValTool/issues/887

[mo-g]

I'm assuming (not knowing offhand) that dask doesn't have any excessively complex dependencies of it's own?

Just thinking of installs on older platforms like EL7, SLES11, etc.

[bouweandela]

The dask unit tests are running on Travis with Trusty Tahr, released in 2014, but they use conda to install their dependencies.

[bouweandela]

The iris unit tests are also failing for the master branch.

[bjlittle]

@bouweandela Could you rebase please? We've now merged back out 2.2.x branch onto master after the recent 2.2.1 release

[bjlittle]

@bouweandela I'm going to make a PR on your branch (for this PR) to add some tests, is that okay?

[bouweandela]

Sure! I hadn't realized that there were separate tests for lazy and non-lazy operators.

[valeriupredoi]

greetings iris peeps 😁 Is this going to be part of the iris=2.3 release? 🍺