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.

[RobertWilbrandt]

For the record: I am also currently using this API for a tool change system that builds a composite URDF at runtime depending on attached tools. I don't require access to tinyxml and would be fine with a string export.

I'd say something like urdf::exportModel would conceptually make sense: parseUrdf converts URDF to a model and this function should go the opposite direction. I'd be happy to create a PR with this in the near future.

[andreasBihlmaier]

For sdf_to_urdf, I would also be interested in a replacement for the deprecated exportURDF() function

[chhtz]

This could also be something like exportURDFtoStream, with an additional std::ostream& argument (in case someone wants to write the output to a file, there is no need to store it in a string first -- and to generate a string, one could stream it into a stringstream).

N.B. It would have been nice not to remove deprecated functions, as long as there are pending issues regarding having an alternative ...