allow to order fractional units

[wvengen]

When buying large packages, a foodcoop may want to subdivide it. Currently, that can be done by adjusting the unit, then manually recomputing the unit_quantity and price. This could be much easier when there would be a unit_divide field, indicating in what units members can order.

This is work in progress on branch uicleanup-new-grouporder-subdivide.

• support non-integer amounts in data model
• include ruby-units for unit computations + configure units
• add subdivision to article edit screen
• reintroduce edit boxes in member order screen
• support fractional units in member order screen
• check negative entry in member order @wvengen
• show price/kg when sensible in member order and admin order screen (using helper) @wvengen
• unit tests for the use of unit_divide (see also Get full test coverage (using Travis CI) foodcoops/foodsoft#120) @tg-x
• fix any close order and distribute issues
• integration test(s) for subdivided order @tg-x
• make balancing screen work (might work already)
• test that everything (still) really works ok

would be nice / need to do in the future:

• check that performance doesn't get much worse
• rebase on upstream (involves some changes)
• open upstream pull request - see Edit article user-interface update + unit_subdivision foodcoops/foodsoft#209
• change confusing gray box style of subdivide box in edit article screen
• use ruby-units in Article#convert_units

[wvengen]

this is mostly done, what remains is finishing unit and integration tests

[wvengen]

This is stuck, by the way. May need a data-model redesign (see mails 1 and 2).