support export URDF

[briansoe66]

I would like to use the export functions. However, they are deprecated, so that the tinyxml2 dependency is private, as described in: PR #190

I propose changing the API, so that export is available does not use tinyxml2. If this is acceptable, I can create a PR.

URDFDOM_DLLAPI std::string  exportURDF(ModelInterfaceSharedPtr &model);

URDFDOM_DLLAPI std::string  exportURDF(const ModelInterface &model);

[clalancette]

In general, I am OK with that, but you are going to have to call them something different since we want to leave the existing deprecated ones in place for now . That's because in C++ you can't have an overload that differs only in the return type.