Database writemany

Ready to be merged

Added _writemany function so that several rows can be written at once. This gives a significantly faster transfer of database rows to a remote postgreSQL server. For transfers between local databases however, using the writemany function does not give a noticeable speedup.

Also, setting up the values tuple is now handled by a separate function _get_values, that both _writemany and _write calls.

Futhermore, _convert_tuple_to_row has been updated to avoid hardcoded indices.