Research into the cost of std::floor and family in performance-critical code

[glpuga]

Feature description

As part of #199 it was found that std::floor is expensive. When used in a tight loop, this extra cost can be non-negligible.

This ticket is about looking into other uses of std::floor and similar FP functions (std::trunc, etc) in Beluga, looking into their potential impact on performance and analyzing possible alternatives.

See these comments for reference:

• Optimize likelihood field model runtime performance #199 (comment)
• Optimize likelihood field model runtime performance #199 (review)

And the discussion here: https://stackoverflow.com/questions/824118/why-is-floor-so-slow/825916#825916