adding missing implementation of operator=(const NmssmSoftsusy&)

Hi Ben,

I found that `operator=(const NmssmSoftsusy&)` is not implemented.  This will lead to linking problems when a user tries to copy one `NmssmSoftsusy` object to another one using this operator.

This pull request will add the implementation of `operator=(const NmssmSoftsusy&)`.

Best regards,
Alex