Add functions for checking, repairing, and compacting database

[leifeld]

Both in DNA and rDNA, it would be desirable to have a function that...

• checks if all tables are present in the database and adds tables if necessary
• checks if all necessary primary key entries are present that are referenced in other tables and adds them if necessary
• compacts the database to free space
• corrects any problems with statement positions outside the document text length.

In DNA, this could be executed automatically when a database is loaded and closed and when DNA is closed. In rDNA, this could be executed automatically when dna_init is used. That is, if this does not take too long.