SkhcDb.MusicPropertySourceOperations on music's property
type of music's property
init music_id key value creates a relation by associating music_id with key and value.
equal m1 m2 tests whether m1 equals m2. All fields are used for the test.
insert db property inserts property inside db.
val properties :
Database.t ->
int64 ->
(SkhcMetadata.Metadata.Values.t SkhcMetadata.Metadata.Properties.t,
Sqlite3.Rc.t * string)
resultproperties db music_id returns all the properties associated with music_id in db.