Annotation of CellML elements outside of the libCellML object model

[dladd]

When introducing annotation identifiers in #123 , the decision was made to start with elements that have object representations in libCellML. However, at the time of writing, the Specification allows for all XML elements within a document to be annotate-able:

Any element information item in the CellML namespace MAY contain an attribute information item with local name id, in the CellML namespace. This attribute information item SHALL be treated as having attribute type ID, as defined in section 3.3.1 of XML 1.

Support for annotating elements which don't have direct representations in libCellML (e.g., connections or encapsulations) is a feature we'll want to support down the line but is not an immediate priority.