Problem: In the <indicator> element, there is currently no way to identify precisely which indicator is being reported on, which makes it impossible to aggregate data coming from disparate sources.
Proposed change: Support (and eventually require) at least one <reference> element under <indicator>.
One or more `<reference>` elements must be listed under the `<indicator>` element. Like the `<sector>` element, this has a `@vocabulary` attribute and a `@code` attribute. The `@vocabulary` attribute references a known indicator library, like the [U.S. Foreign Assistance Framework](http://www.state.gov/f/indicators/) or the [WHO Indicator Registry](http://www.who.int/gho/indicator_registry/en/). The `@code` attribute is a unique identifier within that vocabulary. As with the `<sector>` element, `vocabulary="999"` denotes a private vocabulary.
More than one `<reference>` element can be listed, in cases where the same indicator is referred to by multiple codes within different vocabularies (e.g. an internal code and a standard code).
<reference vocabulary="999" code="1.2.3" />
<reference vocabulary="4" code="A3.2-1" />