fdo#74112 Added properties to store chart display unit information in import.

   - added display unit properties.
   - added axis display unit converter class.

Change-Id: Iaa1f9a175b1109c8da7321a1567a04fb981568d1
Reviewed-on: https://gerrit.libreoffice.org/7720
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>