data integrity from joins

[f-hafner]

• unique links
• no duplicate records within id-year etc

how to implement:

res <- dbSendQuery(
  conn = con,
  statement = "select type, tbl_name, sql from sqlite_master where type = 'index'"
)
dbFetch(res) # returns a df with the relevant information
dbClearResult()

# extract the index columns and whether it is unique

[f-hafner]

use the same approach for copying indexes from the production to the mockdb

[f-hafner]

the first part is implemented in commit caf9561. Closing this, see #13 for copying indexes.