DD: Address clang warning
DD: Address clang warning

In file included from DD.cpp:18:
../../include/geos/math/DD.h:122:9: error: definition of implicit copy assignment operator for 'DD' is deprecated because it has a user-declared copy constructor [-Werror,-Wdeprecated-copy]
        DD(const DD &dd) : hi(dd.hi), lo(dd.lo) {};
        ^
DD.cpp:389:15: note: in implicit copy assignment operator for 'geos::math::DD' first required here
            r = r*r;

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