Re: gaussian distribution pgbench -- part 1/2

pgbench with gaussian & exponential, part 1 of 2.

This patch is a subset of the previous patch which only adds the two
new \setrandom gaussian and exponantial variants, but not the
adapted pgbench test cases, as suggested by Fujii Masao.
There is no new code nor code changes.

The corresponding documentation has been yet again extended wrt
to the initial patch, so that what is achieved is hopefully unambiguous
(there are two mathematical formula, tasty!), in answer to Andres Freund
comments, and partly to Robert Haas comments as well.

This patch also provides several sql/pgbench scripts and a README, so
that the feature can be tested. I do not know whether these scripts
should make it to postgresql. I would say yes, otherwise there is no way
to test...

part 2 which provide adapted pgbench test cases will come later.

-- 
Fabien.