BUG: Identified bug with numpy 1.7 ufunc operators

[cpelley]

Replaces messy #1052

[cpelley]

Ready for review

[cpelley]

I cannot point this to 1.6.x as it requires enhancements which are in master (masked array equality)

[esc24]

Note that this can also be fixed by changing https://github.com/SciTools/iris/blob/master/lib/iris/analysis/maths.py#L537 from

operation_function(new_cube.data, out=new_cube.data)

to

new_cube.data = operation_function(new_cube.data)

It's a little cleaner perhaps. I'm not sure of the performance/memory impact of the two approaches.

[cpelley]

Note that this can also be fixed by changing...

Thanks for your interest @esc24
The workaround provides the same functionality as that intended (in-place operations).
At a glance, the workaround you provide, though cleaner, is not in-place, so twice as much memory will be utilised until it replaces the original array. This would restrict the use to data which is half the size which it can currently handle.

[cpelley]

I'm closing in favour of targeting the v1.6.x branch (made possible after #1079)

[esc24]

Good point @cpelley.