Exports to xls and ods formats

This now has functions to export a data set to Excel (.xls but not
.xlsx) and LibreOffice (.ods) format.
Exports to CSV

This module now exports to CSV.
Removed bug

The datagrid's meta was reset when importing data. This led to a
corruption that hindered saving. The meta data are no longer reset.
Set 'decplaces' to initialise to empty string

Initialising to None caused problems when saving with ezodf. Changing
to an empty string avoids this.
Grid no longer saves, just gets modules to do work

The datagrid used to try and save by itself. It now just instructs the
modules to do the work for it. Some other changes were made to ensure
that files open properly too.