Return CML as a String rather than a XOM element
OPSIN currently returns CML (Chemical Markup Language) as a XOM element. Changing this interface to a String would allow OPSIN's dependency on XOM to be removed and would provide access to the CML without knowledge of a 3rd party library. On the downside the signature of parseToCML would be changed. This change would be part of a v2.0 release.