[MONDRIAN-2107]  Incorrect segment rollup with overlapping segments.

There was the possiblity of creating invalid segments if rolled up segments overlapped and/or were unconstrained for one or more columns.
This commit adds checks for cases where more than one segment has an unconstrained column (i.e. requestedValues=null) and merges the value sets.  This avoids
"losing" values.  Also added verification that cells being added to a rollup haven't already been added (if there's an overlap).
Merge pull request #329 from mkambol/mondrian-2107

[MONDRIAN-2107]  Incorrect segment rollup with overlapping segments.