AthContainers: Rework jagged vector elements to only store one index.

We were storing each jagged vector element as a pair of (begin, end) indices. This is redundant, because the end index of one element is the same as the begin index of the following element.

It was done like this so that we could treat jagged vector elements as independent objects, as expected for xAOD.

However, users don't interact with jagged vector element object directly, only through the Accessor classes.

So it turns out that we can indeed remove one of the indices without really giving up anything.

Now, an element contains only the end index. The begin index has to be gotten by looking at the previous element (and we need to special-case the fact that the begin index of the first element is 0).