Add a fmod function that can give the modulo of two real numbers

[Sander-Toonen]

See this thread on Stackoverflow:

Before PHP 7.2, bcmod does not handle real numbers well. There is no alternative function to PHP's native fmod in BC Math.

I added a function called BC::fmod that can give the remainder after "divving" two real numbers. In theory this function can replace the ''mod''-function, especially since it mimics the behaviour of bcmod from PHP 7.2. However creating a separate function minimises the risk of introducing problems with users that might falsely rely on the current behaviour.

[krowinski]

Tx!