Text consistency changes

ppO2 represented with 2 as subscript.
Special l symbol for litres.
Capitalisation to bring in line with existing style.

Signed-off-by: Tim Wootton <tim@tee-jay.demon.co.uk>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Support timedepth mode on UDCF import

Fixes #756

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>