Modified subject, experiment and assessor code to allow specification of labels when sharing to another project

pyxnat's implementation of sharing into projects did not support setting labels. Subjects would therefore appear in the shared-to project with xnat's unique identifier as the label instead of the original label, a source of confusion to the user. I have added  a new function sharelabel(self,project,label=None)to each of Subject, Experiment and Assessor. The value of label defaults to the label in the original project but can be set explicitly.
Example code for a db which will contain data from a number of teams in separate projects, each team being supposed to use a unique two-character label prefix:
  mp=lox.select.project('IMI_WP2').subjects()
  sp=mp.sharing(['sharedproject'])
  splist = sp.get()
  for xx in mp:
    if xx.id() in splist:
      print('Already shared: '+xx.label())
    else:
      print('Sharing: '+xx.id()+' '+xx.label())
      try:
        xx.sharelabel('sharedproject')
      except:
        print('Share failed, trying extended label')
        xx.sharelabel('sharedproject','CR'+xx.label())