Add Element::GetElementCount()

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/156)) by Martin Pecka (Bitbucket: [peci1](https://bitbucket.org/%7B41691c43-4ae0-49ba-ba45-3c36439f5af2%7D/)).**

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The `Element` class is missing method `GetElementCount`, and probably also `IsEmpty`.

The first one should just return `this->dataPtr->elements.size()`, the latter could probably return true if the element has no attributes and no child elements.

Adding at least the `GetElementCount` method would complete the API, where `GetAttributeCount` and `GetElementDescriptionCount` already exist.

Without this method, it is cumbersome to verify whether an element has any child elements. This is what I've come up with (it contains a completely useless object creation, which may get expensive):

auto isEmpty = element->GetFirstElement() != sdf::ElementPtr();

The cumbersomeness of this approach is elevated by the fact that `GetFirstElement` has no documentation at all, so there's no place that'd promise that `GetFirstElement` will always return a pointer to an empty element and not e.g. `nullptr`.

Start by locating the Element class and comparing its existing GetAttributeCount and GetElementDescriptionCount APIs. Confirm where the public declaration and implementation belong, then verify that the requested child-element count behavior is covered; treat IsEmpty as optional because the report requires at least GetElementCount.

Written by the indexing model from the issue text.