Changed const reference to non-const reference to fix Boost.Serializa…

[nilay2207]

// Added BOOST_CLASS_EXPORT macro to register polymorphic class with Boost.Serialization class BOOST_SYMBOL_VISIBLE MyPolymorphicClass : public MyBaseClass { // Class definition here }; BOOST_CLASS_EXPORT(MyPolymorphicClass)

[mclow]

There are a couple of problems with this PR:

• It's targeted at master. We never commit directly to master; instead we commit to develop and then merge to master after the tests have passed.
• this looks like it should be committed to Boost.Serialization, not the main boost project. https://github.com/boostorg/serialization