Bug: a88/e21

It is possible to save the effective sensor lengths as part of the
2G output format, with the column headings "X responce", "Y responce",
and "Z responce" (sic). This doesn't seem to be commonly done, but
if these fields are available it could remove the need to manually
enter sensor lengths in the Preferences dialog.

One problem with these values: they are only written to two decimal
places in the DAT file, whereas in the 2G software they are stored
to (at least) four decimal places. So sensor lengths read from the
file will be slightly less accurate. However, the difference would
be fairly small (<0.2% for a sensor length of 5cm) so it probably
won't make much difference in practice.