Automatically detect sdf extension from parsed string

[flferretti]

This PR enable detecting the XML type from the parsed string.

[diegoferigo]

Thanks @flferretti for this contribution! This PR introduces the automatic detection of either the URDF or SDF type when the model description is a string containing the content. Before this PR, in this case the user had to pass a is_urdf argument.

If the model is a pathlib.Path, we read its extension for the file name, and it cannot be wrong. Having some heuristics on the URDF detection also when the passed description is a string, is definitely useful. However, the proposed solution is very fragile, and I suggested an more robust solution.

Furthermore, I'd not change the APIs by removing entirely the is_urdf argument. With this PR, this argument will be useless in most cases, now extended to models passed as strings. Though, there might be edge cases where forcing either SDF or URDF is still necessary, and I'd suggest to leave this is_urdf argument there to address them. We can update the docstring to something like: Force the SDF resource to be treated as URDF if the automatic detection fails.

[diegoferigo]

LGTM! Thanks @flferretti for your contribution.