clang compiler error for VerticalFFAMagnet (-Wabsolute-value)

Summary

Compiler error for VerticalFFAMagnet

Steps to reproduce

Host OS System: Darwin-14.5.0
The C++ compiler identification is: AppleClang
The C++ compiler version is: 7.0.2.7000181

Relevant logs and/or screenshots

src/Classic/AbsBeamline/VerticalFFAMagnet.cpp:82:9: error: 
      using integer absolute value function 'abs' when argument is of floating point type
      [-Werror,-Wabsolute-value]
    if (abs(R[0]) > halfWidth_m ||
        ^
/Users/jsnuverink/Documents/OPAL/fork/src/src/Classic/AbsBeamline/VerticalFFAMagnet.cpp:82:9: note: 
      use function 'std::abs' instead
    if (abs(R[0]) > halfWidth_m ||
        ^~~
        std::abs

Possible fixes

Use std:: namespace