#12 Add Compare dataset, Append Dataset functionality

[Alok Saldanha]

should properly implement the Compare, Append menuu items
should create new window with composite DataModel instead of
modifying existing DataModel

[Alok Saldanha]

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This might be more useful once we have the ability to save CDT files.
Also, this is a case where being able to independant color columns would be
really handy. (see [ a href="https://sourceforge.net/support/tracker.php?
aid=1359347"> 1359347 </a>] )
Maybe it's not such a bad idea.