Encoding relationships between annotations

[jonnybluesman]

In the same JAMS, there might be annotations (with same or different namespace) that refer to other annotations. We should keep track of these relationships in the sandbox, or somewhere more appropriate. This also requires an identifier (in the JAMS file) for each annotation. Examples of possible relationships:

• Same as -- when 2 annotations are basically the same, but they are expressed in different ways.
• Converts -- when 1 annotation is the output of a conversion / simplification / pre-processing.
• Extends or derived from -- when 1 annotation extends another, or is simply derived from another annotation (e.g. a measure of chord complexity derived from a chord progression).

To some extent, there are cases when some of them can be inferred from the annotation metadata (e.g. "Jacopo" has produced 2 chord-harte annotations of the same piece, so, these can be the same?), but preferably, we should make things explicit.