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

Related pull requests:
* ["Improve refinery modelling" on ETdataset](https://github.com/quintel/etdataset/pull/437)
* ["Improve refinery modelling" on ETsource](https://github.com/quintel/etsource/pull/773)