Add utility function to check validity of hex strings.  Added function GetXmlDocument to PremisEntity class.  Added MustOverride LocalIdentifierValue property to PremisEntity and all subclasses.  Added utility function GetMessageDigest to the PremisObjectCharacteristics class to return the value of the digest for a given algorithm.  Modified PremisRights class so that it can contain only a single RightsStatement which is a restriction of the PREMIS schema, but which simplifies some Medusa processing.