Boost factors have to be formated independent from locale

Problem/Motivation

Boost factors have to be formatted regardless of the locale, always use a period, never a comma.

Steps to reproduce

sprintf('%.1f', $boost)

Proposed resolution

The problem is the sprintf with parameter "f" - "the argument is treated as a float and presented as a floating-point number (locale aware). "

On German Websites (locale=de_De) this leads to a boost_term "String|1,0" and this throws a error on the Solr server
-> possible analysis error: For input string: "5,​0"

We have to use the parameter '%.1F' instead