[SP-1364][MONDRIAN-2128][MONDRIAN-2016]  Addressing two issues with the ...

...Aggregate function when a second parameter is specified.  Aggregate() would formerly sum up the value of the second parameter, even if a different aggregator was appropriate (e.g. if a count distinct measure was specified).  If the second parameter is not a base measure we fall back to summing (consistent with SSAS when Aggregate is applied on a non-measures calculated member).

This commit also fixes a related issue involving non-additive measures in context when the 2 argument form of Aggregate is evaluated.  Formerly this would cause the second argument to be ignored.
These changes were merged from Will Gorman's branch:  https://github.com/wgorman/mondrian/commit/e19cda6a2e1b03e2482dca502ffef258e41baa7a
(cherry picked from commit 5ab0031)