migration struggles to fit non-linear study areas into the survival numbers summary (in the report)

[hugomflavio]

Reported by Scott Colborne.

In a situation where the sections do no follow each other linearly, actel will not be able to correctly fill in the numbers in the survival summaries.

I.e. if the sections are like this:

A -> B -> D
A -> C -> D

The summary code needs to be updated to be able to accommodate non-linear sections.

[hugomflavio]

This one is more complex than I was expecting, so I won't be able to fix it in time for the next release.

[hugomflavio]

if.last.skip.section will need to be updated too