[MONDRIAN-1519]  Median from set of one member is returned as 50% from measure value.

In the case of single value arrays, the percentile function was returning p*value, which is inconsistent with most other implementations of Percentile (e.g. Excel's).
(cherry picked from commit c3956f2)
Merge pull request #325 from mkambol/lag1519

[MONDRIAN-1519]  Median from set of one member is returned as 50% from measure value.