Fix: [ bug #1792 ] Users are not allowed to see margins module index page when no product view permission is enabled

http://php.net/manual/en/language.basic-syntax.phptags.php

This solve bug of rounding with total price when we need a unit price.
This solve lso bug of rounding on non decimal basis that we don't want.
Also this remove calling database when not usefull so make possible to
call this function into fetch (this remove the FIXME not into comment
so).

upgrade were not yet received. So we prepare for GPL by uniformizing
licence text keys to GPL-3+. Will move later to AGPL if all answers are
positive.

commits !

Qual: Because params were often missing in calculate_price(), params are
now mandatory.