improve slack distribution among multiple gens at slack bus

Currently, when there are multiple generators at the slack bus, MATPOWER's power flow considers the first generator at the bus to be the slack generator, taking up all of the slack. However, I'm not sure if this convention is documented anywhere and it is probably not the behavior intuitively expected by most users.

I propose to update MATPOWER's handling of slack allocation among multiple generators at a slack bus in the following way:

1. If area participation factors (APF column of gen matrix) are provided, split the slack according to the proportions indicated by the relative participation factors.
2. If area participation factors are not provided (i.e. set to 0), the slack will be distributed according to the size of the generator. I.e. for identical generators it will be distributed evenly.

This also needs to be mentioned explicitly in the documentation.