Fix unable to create an AR

2014-07-24 12:31:21 ERROR Zope.SiteErrorLog 1406197881.570.422280493169 http://localhost:8080/Plone/clients/client14-22/analysisrequest_submit
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.lims.browser.analysisrequest.add, line 288, in __call__
  Module bika.lims.utils.analysisrequest, line 40, in create_analysisrequest
  Module bika.lims.utils.samplepartition, line 36, in create_samplepartition
KeyError: 'part_id'