Link identification

When this is on, then two or more XGobis that were initiated with
the same data (or have the same number of rows) *or* have the same
value of `fname.nlinkable' are linked for identification.  That is,
if one XGobi is in Identify, any case labelled in that XGobi's
plotting window are promptly labelled in the other XGobis.

This is most frequently used when the XGobi plotting windows
display different views of the same data.

Note:  For computational efficiency, the receiving XGobis should be
in XYPlot, Plot1D, or paused in Rotate or Tour.

This can be set in the XGobi resource file.  The default is

*linkIdentify:   True