Disallow flexible links from having a negative share

Closes #684
Remove debug lines from the ETSource refresh method
Renamed final_demand_cbs into final_demand_group
Merge pull request #688 from quintel/rename_final_demand_cbs

Renamed final_demand_cbs into final_demand_group
Converter detail page links to node and edge documents
Tighter restrictions on the Rubel version
Update Refinery to quintel/refinery@f29ead1

Fixes a bug in the FromPartialSlot strategy.
Slot calculation should consider reverse shares

When a slot has a mixture of "forward" and "reversed" share links, the
reversed shares are not considered when calculating the demand of the
slot due to them being considered "active" links.

Therefore, slots with such a mixture need to have their "external_value"
altered to account for the as-yet-uncalculated reverse shares.

Ref #689
Merge pull request #690 from quintel/reverse-share-comp

Slot calculation should consider reverse shares
converter_positions.yaml -> converter_positions.yml
Warn a user when converter positions are unsaved

If the user has edited converter positions, but forgot to save the YAML
file, they will receive a message reminding them to do so.

Closes #680
Remove unnecessary question

We don't need to ask the user's intent in the converter position
confirmation; the browser pop-up already asks it.
Update debugger to v1.6.6

Adds compatibility with Ruby 2.1.1.
Remove debug line from Slot
Update to Rails 4.0.3
Added new area variable to engine
Merge pull request #691 from quintel/netork_area_constants

Added new area variable to engine
Removed peak_load attribute
Removed residual_value
Structured the chemical industry