Support for structural annotations added.  Reader and writer modules preserve structural xml annotations nested in SBOL files as non-SBOL namespaced elements
Restored cleanup routine in readDocument
Reader module can pass xml namespaces to writer module through the intermediate Document->xml_doc object..
Cleaned up
Writer module now inserts correct xml namespaces when outputting a document with structured xml annotations.
Added getters for structural annotations to the base object SBOLObject
Structural annotations are now supported by each of the four SBOL classes: DNAComponent, DNASequence, SequenceAnnotation, and Collection.
Cleaned up.