#95 support SOUR|DATA|EVEN|DATE

[Nils Meier]

A Date showing up in a Source is a special bugger that
has to get some special data handling:

n @<XREF:SOUR>@ SOUR {1:1}
+1 DATA {0:1}
+2 EVEN <EVENTS_RECORDED> {0:M}
+3 DATE <DATE_PERIOD> {0:1}