Patch for adding DATACUBE operator

Hi!

We have added the CUBE operator for PostgreSQL. Please find the
attached patch.

Another thing to note is that the file datacube.c should
be placed in src/backend/tcop/ and datacube.h should be in src/include.

The syntax of the query is

SELECT <field list><aggregate list>
INTO <destination table>
FROM <table expression>
WHERE <search condition>
GROUP BY <aggregate list>
HAVING <search condition>
WITH CUBE;

An example along with the output is provided in the
README.datacube file. Kindly have a look. Let us know your response.

Srikanth M
Sumit Kumar Mukherjee