Const accessors in sdformat.

[osrf-migration]

Original report (archived issue) by Alejandro Castro (Bitbucket: amcastro0622).

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We had a use case in our Drake code base in which we only needed const access to an sdf::Element. Right now the current API only offers sdf::Element::GetElement() which returns a mutable shared pointer to a mutable element object.
Ideally we'd also like having const Element& GetElement(const std::string&).
Most likely similar const access would be needed in other places.