Remove tinyxml2 from public API

[scpeters]

We are upgrading from tinyxml to tinyxml2 in #186 (resolving #185), which will be included in the next major release. An alternate approach to replacing tinyxml2 in #178 also made a change to redact tinyxml2 from the public API, so we should apply that change as well after #186 is merged.

[moooeeeep]

Did you consider to try to hide the tinyxml2 types with a typedef? Maybe this would help client code with the next upgrade, as they would just continue to use urdf::XMLDocument.

(untested) Example:

namespace urdf{
  using XMLDocument = tinyxml2::XMLDocument;
  ...
  URDFDOM_DLLAPI XMLDocument*  exportURDF(ModelInterfaceSharedPtr &model);
  ...
}

[clalancette]

Did you consider to try to hide the tinyxml2 types with a typedef? Maybe this would help client code with the next upgrade, as they would just continue to use urdf::XMLDocument.

Yeah, we could do that. Honestly, I suspect that all of the APIs that take tinyxml2 APIs are unused, so I'm considering marking them deprecated and eventually removing them.