Support more study-level DDI-C fields

• Support /codeBook/stdyDscr/citation/distStmt/distDate
  • Add Study.distribution_dates with description attribute.
• Support /codeBook/stdyDscr/method/dataColl/resInstru
  • Add Study.research_instruments system_name, uri and description attributes.
• Support /codeBook/stdyDscr/dataAccs/useStmt/conditions/@elementVersion by mapping it to Study.data_access_descriptions.attr_element_version
  • Add element_version attribute Study.data_access_descriptions.
• Support /codeBook/stdyDscr/stdyInfo/sumDscr/collDate CDATA by mapping it to Study.collection_periods.attr_description
  • Add description attribute to Study.collection_periods.