Add a way of tracking the first Individual parsed

[frizbog]

The GEDCOM spec does not have any defined mechanism for specifying who the "primary" or "root" or "home" person is (the primary person of interest in the tree), although many genealogical software tools have a way of "pinning" an individual as the "home person" or the default person to go to when displaying the contents of the file.

Some tools use custom tags to indicate who this primary person is, but an alternative that's in common use is to anchor things to the first Individual listed in the file.

If the parser could track who that individual is, and then have that available somehow in the data model, this would enable tools using gedcom4j to pick this first person and start displaying tree data from there.