sync should add period_startdate field to allow sorting by year

[OriHoch]

reproduction

• sync some photoUnits from clearmash
  • the photoUnits have fields containing estimate of when they were taken
  • this estimate could be a range, e.g. 1960 - 1969
  • in BHP the start date was in field UnitPeriod.PeriodStartDate
• look at the resulting data in Elasticsearch

expected

• should have an attribute period_startdate containing an Elasticsearch date field with the start date
• in case start date is only a year, set it to Jan 1 of that year

actual

• doesn't exist as a root attribute
• this data might be in the parsed_document attribute for Clearmash documents

[OriHoch]

the format of the field should be standard ISO date/time in UTC:

for example:
1950-01-01T00:00:00Z

[OriHoch]

@Libisch are you working on it? if not, please un-assign yourself and move back to new issues

[Libisch]

#66 merged