Fix that FromPartialSlot ignored the slot share

When calculating a node's demand using the FromPartialSlot strategy --
which attempts to determine the demand from edges, some of which have a
demand, and some which have a share -- the resulting demand was not
adjusted to account for the share of the slot.

This was not a problem previously, since this strategy was only used to
calculate nodes with a single input or output slot where the share was
1.0.

Ref quintel/etengine#689.