range_agg with multirange inputs

Here is a patch adding range_agg(anymultirange). Previously range_agg 
only accepted anyrange.

Here is a bug report from last month requesting this addition:

https://www.postgresql.org/message-id/CAOC8YUcOtAGscPa31ik8UEMzgn8uAWA09s6CYOGPyP9_cBbWTw%40mail.gmail.com

As that message points out, range_intersect_agg accepts either anyrange 
or anymultirange, so it makes sense for range_agg to do the same.

I noticed that the docs only mentioned range_intersect_agg(anyrange), so 
I added the anymultirange versions of both on the aggregate functions page.

I also added a few more tests for range_intersect_agg since the coverage 
there seemed light.

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com