autodb 2.3.1

First release to CRAN.

Improvements

Added a convenience function called df_records, for converting a data frame into a list of row values. These are sometimes more useful than data frames, e.g. for checking which rows of a data frame are present in another one.
Some performance improvements. Main improvements:
- Improved the run time of database for larger data sets, specifically the validity checks for the data satisfying the foreign key references.
- Improved the run time of create for relation_schema and database_schema, by removing validity checks. If the input is valid, these are redundant.
- Improved the run time of autodb, by skipping removal of extraneous attributes. This is done on the results of discover, so there won't be any.