struct2xml

'struct2xml': transform a matlab structure to a xml tree.
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 each field with char string or num vector is transformed into a corresponding  xml element
 each field with a matrix containing n lines is transformed into a xml element repeated n times 
 WARNING: PROBLEM WITH HIERARCHICAL structures
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 OUTPUT:
 t: xmltree reproducing the structure of Object
 type 'save(t)' to visualize the xml text and save(filename,t) to save it in a file

 INPUT:
  Object: matlab structure, possibly hierarchical
  t: optional input xml tree in which a new branch needs to be appended
  root_uid: optional uid of the xml element under which the new subtree must be appended