Allow EXPLAIN to indicate fractional rows.

Allow EXPLAIN to indicate fractional rows.

When nloops > 1, we now display two digits after the decimal point,
rather than none. This is important because what we print is actually
planstate->instrument->ntuples / nloops, and sometimes what you want
to know is planstate->instrument->ntuples. You can estimate that by
multiplying the displayed row count by the displayed nloops value, but
the fact that the displayed value is rounded makes that inexact. It's
still inexact even if we show these two extra decimal places, but less
so. Perhaps we will agree on a way to further improve this output later,
but for now this seems better than doing nothing.

Author: Ibrar Ahmed 
Author: Ilia Evdokimov 
Reviewed-by: David G. Johnston 
Reviewed-by: Amit Kapila 
Reviewed-by: Vignesh C 
Reviewed-by: Greg Stark 
Reviewed-by: Naeem Akhter 
Reviewed-by: Hamid Akhtar 
Reviewed-by: Tom Lane 
Reviewed-by: Andrei Lepikhov 
Reviewed-by: Guillaume Lelarge 
Reviewed-by: Matheus Alcantara 
Reviewed-by: Alena Rybakina 
Discussion: http://postgr.es/m/603c8f070905281830g2e5419c4xad2946d149e21f9d%40mail.gmail.com