[osis-core] References to elements with multiple identifiers in an osisID

[Todd Tillinghast]

1) For cases where an element has multiple identifiers in an osisID, a
reference to any ONE of the identifiers will be a reference to the
entire element.

2)  For cases where an element has multiple identifiers in an osisID,
and a reference specifies the grain, the grain will begin counting at
the beginning of the element.

3) For references with the grain specified and the element with the
referenced osisID is split using a splitID element, the counting
relative to the grain begins with the first element of the split and
continues in document order through all of the elements with the same
splitID.

4) For references with the grain specified, if multiple elements exist
in the same document with referenced identifier the reference is an
erroneous reference. 

Todd