Use std::abs instead of abs and fabs

[richelbilderbeek]

Currently, the code contains calls to abs and fabs, to get the absolute value of a number. These functions, however, are not C++ functions: these are C functions. CPL.1: Prefer C++ to C. In C++, there is std::abs to get the absolute value of any type of number.

Look for abs and fabs and replace these both by std::abs

[Luxwor]

Fixed