Chi square

The test pass, though clang has a warning I don't really get.
```
include/ef.gy/statistics.h:122:26: warning: inequality comparison result unused [-Wunused-comparison]
            for (; begin != end, measurement_begin != measurement_end; ++begin, ++measurement_begin)
                   ~~~~~~^~~~~~
include/ef.gy/statistics.h:133:19: note: in instantiation of function template specialization 'efgy::statistics::chi_square<double,
      __gnu_cxx::__normal_iterator<const double *, std::vector<double, std::allocator<double> > > >' requested here
           return chi_square<T>(values.begin(), values.end(), measurements.begin(), measurements.end());
                  ^
src/test-case/statistics.cpp:107:17: note: in instantiation of function template specialization 'efgy::statistics::chi_square<double>' requested
      here
    auto chi0 = chi_square(v8, v8_measurement);
                ^
include/ef.gy/statistics.h:122:26: note: use '|=' to turn this inequality comparison into an or-assignment
            for (; begin != end, measurement_begin != measurement_end; ++begin, ++measurement_begin)
```

#2