Unlike system_imports.py and system_exports.py, which
deal with whole tables via COPY, this allows us to
export / import *either* whole tables or specific
columns only.
The data will be exported to text files containing
the selected columns + the primary key columns for
the table.
When importing, those tables for which a selection
of columns was exported must already be populated.
The import process will overwrite the data of the
non primary key columns it knows about. If whole
tables are exported, on the other hand, when
re-importing rows will be appended rather than
updated. It is the user's responsibility to make
sure that this will not cause any conflicts.
