Const accessors in sdformat.

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/188)) by Alejandro Castro (Bitbucket: [amcastro0622](https://bitbucket.org/%7Bf78f4af9-eed7-49c4-b222-5b3dc10d5093%7D/)).**

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We had a [use case in our Drake code base](https://reviewable.io/reviews/robotlocomotion/drake/8828#-LD2bp_9--hRPdZXTAQL) 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.

Start at the sdf::Element::GetElement() API described in the issue and inspect related accessors for places where const access is currently unavailable. Add the requested const Element& access and determine whether similar const accessors are needed elsewhere; done means const-only callers can access elements without mutable pointers.

Written by the indexing model from the issue text.