Don't save cylinder start/end pressures unless set by hand

Don't save cylinder start/end pressures unless set by hand

Keep the sample pressure start/end data separate from the overall
cylinder start/end data - and clean the overall cylinder start/end data
if it matches the samples exactly to avoid the redundancy.

This breaks all the SAC calculations etc, which expect the cylinder
pressures to always be in the cylinder data.  I'll fix that up
separately.

The reason for this is that we really want to keep the manually entered
data separate: the pressure plotting doesn't need the confusion, and
considers end-point data (with interpolation) very different from sample
data.  Also, we do not want to pollute the xml save-file with data that
is computed.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>