Support more DDI-C elements

• Map DDI-C /codeBook/stdyDscr/citation/distStmt/distDate to Study.distribution_dates.
• Map DDI-C /codeBook/stdyDscr/method/dataColl/resInstru to Study.research_instruments.
• Map DDI-C /codeBook/stdyDscr/dataAccs/useStmt/conditions/@elementVersion to Study.data_access_descriptions.attr_element_version
• Map DDI-C /codeBook/stdyDscr/stdyInfo/sumDscr/collDate CDATA to Study.collection_periods.attr_description.