Move calculation of data out of its enumeration in NoSmallGapsStrategy

[mmmcaffeine]

We have a couple of methods in NoSmallGapsStrategy of FindAllIndices and FindGaps. The latter is returning a tuple with information about the gap i.e. the index of the first cell, and the length. However, when we enumerate the return value of this method in Execute we're calculating more things about the gap! Specifically this is the index of the last cell. We should keep Execute to only doing the enumeration, and move all calculations about the gap into the FindGaps method.

We could achieve this by changing the return tuple to either (int StartIndex, int EndIndex, int Length) or (Range Range, int Length) The latter would be nice as we could pass it directly to the array indexer when checking all the cells in the gap are undetermined. That would also mean we'd have to access Range.Start and Range.End in other places, which isn't quite as neat 🤷‍♂️