A typed and referenced property from an external resources.

Can be attached to Phylogeny, Clade, and Annotation objects.

:Parameters:

value : string
    the value of the property
ref : string
    reference to an external resource, e.g. "NOAA:depth"
applies_to : string
    indicates the item to which a property applies to (e.g.  'node' for
    the parent node of a clade, 'parent_branch' for the parent branch of
    a clade, or just 'clade').
datatype : string
    the type of a property; limited to xsd-datatypes
    (e.g. 'xsd:string', 'xsd:boolean', 'xsd:integer', 'xsd:decimal',
    'xsd:float', 'xsd:double', 'xsd:date', 'xsd:anyURI').
unit : string (optional)
    the unit of the property, e.g. "METRIC:m"
id_ref : Id (optional)
    allows to attached a property specifically to one element (on the
    xml-level)