Re: gaussian distribution pgbench

Please find attached 2 patches, which are a split of the patch discussed
in this thread.

(A) add gaussian & exponential options to pgbench \setrandom
the patch includes sql test files.

There is no change in the *code* from previous already reviewed
submissions, so I do not think that it needs another review on that
account.

However I have (yet again) reworked the *documentation* (for Andres Freund
& Robert Haas), in particular both descriptions now follow the same
structure (introduction, formula, intuition, rule of thumb and
constraint). I have differentiated the concept and the option by putting
the later in <literal> tags, and added a link to the corresponding
wikipedia pages.

Please bear in mind that:
1. English is not my native language.
2. this is not easy reading... this is maths, to read slowly:-)
3. word smithing contributions are welcome.

I assume somehow that a user interested in gaussian & exponential
distributions must know a little bit about probabilities...

(B) add pgbench test variants with gauss & exponential.

I have reworked the patch so as to avoid copy pasting the 3 test cases, as
requested by Andres Freund, thus this is new, although quite simple, code.
I have also added explanations in the documentation about how to interpret
the "decile" outputs, so as to hopefully address Robert Haas comments.

--
Fabien.