Fix clang compile errors/warnings

In file included from src/FiberTraceFunction.cc:1:
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:2690:3: error: static_assert failed
      due to requirement
      '__is_cpp17_forward_iterator<ndarray::detail::BinaryOpIterator<ndarray::ArrayRef<unsigned long, 1, -1>,
      ndarray::ArrayRef<unsigned long, 1, -1>, ndarray::detail::MinusTag::ScalarFunction<unsigned long,
      unsigned long> > >::value' "std::minmax_element requires a ForwardIterator"
  static_assert(__is_cpp17_forward_iterator<_ForwardIterator>::value,
  ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/algorithm:2741:19: note: in instantiation of
      function template specialization
      'std::__1::minmax_element<ndarray::detail::BinaryOpIterator<ndarray::ArrayRef<unsigned long, 1, -1>,
      ndarray::ArrayRef<unsigned long, 1, -1>, ndarray::detail::MinusTag::ScalarFunction<unsigned long,
      unsigned long> >, std::__1::__less<unsigned long, unsigned long> >' requested here
    return _VSTD::minmax_element(__first, __last,
                  ^
src/FiberTraceFunction.cc:29:34: note: in instantiation of function template specialization
      'std::__1::minmax_element<ndarray::detail::BinaryOpIterator<ndarray::ArrayRef<unsigned long, 1, -1>,
      ndarray::ArrayRef<unsigned long, 1, -1>, ndarray::detail::MinusTag::ScalarFunction<unsigned long,
      unsigned long> > >' requested here
    auto const leftMinMax = std::minmax_element(leftPixels.begin(), leftPixels.end());
                                 ^
1 error generated.
scons: *** [src/FiberTraceFunction.os] Error 1
scons: building terminated because of errors.