[WIP] Preliminary support for URDF export from iDynTree::Model

[traversaro]

Missing features:

• Handle geometries
• Handle sensors
• Handle joint axis do not pass through the origin of the child link frame
• Handle Propagate to non-KDL classes the improved definition of URDF "fake link" introduced in https://github.com/robotology/idyntree/pull/246 #247
• Expose in bindings
• Have a nice class with a nice interface that in the future can support more options, like we have iDynTree::ModelLoader for loading models

However it is working fine, feel free to do a preliminary review if you want @francesco-romano @fiorisi @diegoferigo

[traversaro]

cc @fjandrad

[traversaro]

After working a bit with the libxml2 Tree API (see http://www.xmlsoft.org/examples/index.html#testWriter.c), I think it is sufficiently similar to TinyXML's API so that we could easily migrate this PR to using libxml2. The only caveat is that with libxml2's C-based API is extremely easy to introduce memory leaks, so any such code should be tested and the test should run under Valgrind in the CI. If anyone is interested in working on this, please contact me so that I can provide also some examples of TinyXML --> libxml2 migration.

[traversaro]

Closing this PR has it has been stale quite a while. Work on URDF export will be tracked in #536 .