Fix up small details in input/output

Fix up small details in input/output

Be more careful with FP conversions, and with the Kelvin<->C offset.
And make sure to use the same names when saving as when parsing.

Now when we save a set of dives, then re-load them, and save again, the
second save image is identical to the first one.

Of course, we don't actually save everything we load, so we still do
lose information when we load and then save the result.  But at least we
now don't lose the information that we *do* save.

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