including `cmath` required on macOS

Summary

On macOS we have to include cmath in several files otherwise compilation fails

Steps to reproduce

Compile on macOS with current Xcode

Relevant logs and/or screenshots

/Users/gsell/src/OPAL/src/src/Classic/Filters/RelativeFFTLowPass.cpp:28:12: error: call to 'abs' is ambiguous
        if(std::abs(LD[i]) > max_four_coef) {
           ^~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/include/stdlib.h:132:6: note: 
      candidate function
int      abs(int) __pure2;
         ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/stdlib.h:110:44: note: 
      candidate function
inline _LIBCPP_INLINE_VISIBILITY long      abs(     long __x) _NOEXCEPT {return  labs(__x);}
                                           ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/stdlib.h:112:44: note: 
      candidate function
inline _LIBCPP_INLINE_VISIBILITY long long abs(long long __x) _NOEXCEPT {return llabs(__x);}
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Possible fixes

include cmath

(If you can, link to the line of code that might be responsible for the problem)