Fix negative contributions grouping under Rest issue

[nabilahmed739]

Fixes the way bw2-analyzer treats negative contributions on the AB side: brightway-lca/brightway2-analyzer#18
Note: If the above issue is fixed in bw2-analyzer in accordance with the current fix then this change can be reverted after consideration if even the changes in normalize method in multilca class is not required any more.

fixes #739

[coveralls]

Coverage remained the same at 54.301% when pulling 8169a11 on bugfix-739 into be1113a on master.

[marc-vdm]

Would it be a good idea to add TODOs to places that need to be changed back if this gets changed on the BW side?
Otherwise, looks great!

[cmutel]

@nabilahmed739 I think it is awesome that you have submitted the PR. Thanks!

I have merged your PR against bw2-analyzer, I think this is a better way to fix the underlying problem. I will release a new version today or tomorrow.

[nabilahmed739]

@cmutel Yes, the change in bw2-analyzer would be perfect!
Could you please let us know the new release version of bw2-analyzer which will be compatible with AB? (Please note that AB has not migrated to bw2.5 yet)

[nabilahmed739]

The complete fix for #739 is the combination of bw2-analyzer update to version 0.11.2 and completion of this PR

[marc-vdm]

@cmutel I see bw2analyzer was updated, can we finalize this PR on AB side?

[cmutel]

@nabilahmed739 @marc-vdm All bw2analyzer releases are 2 & 2.5 compatible (or at least they should be!). +1 on merging from me.