Support exporting models to SDF (Simulation Description Format)

[traversaro]

This is a sister issue of https://github.com/robotology/idyntree/issues/481, but tracking the export of SDF (i.e. iDynTree::Model ---> SDF file/string ) rather then the import (i.e. SDF file/string ---> iDynTree::Model).

[traversaro]

Some non-organized notes:

• The official implementation sdformat could be used if the question "Is the DOM read only or can be used also for generation?" tracked in https://github.com/osrf/sdformat/issues/292#issue-620602540 .
• If the answer to the previous question is "No, sdformat DOM is only for reading SDF files, not for writing them", we could either understand if there is some lower level API of sdformat that we could use to generate SDF files, or even roll out our own code down to the XML library, given that by definition the amount of features used during the export are limited, as opposed to the import that should be able to load any model that respect the SDF spec.
• We should understand which version of sdf we want to target. 1.8 contain many convenient features that may be useful especially for model generation, but if we want to target Gazebo Classic we need to stick to 1.7 (see http://sdformat.org/tutorials?tut=roadmap&cat=developers&).

[traversaro]

Even if at the moment no one is interesting in working on this (hence the "Help Wanted" label) this feature could be convenient for users that have their own custom simulation engine for closed loop mechanical structure in high level languages, and may find convenient to export their model to SDF for interoperability with other tools. cc @FabioBergonti