Move more dive computer filled data to the divecomputer structure

This moves the fields 'duration', 'surfacetime', 'maxdepth',
'meandepth', 'airtemp', 'watertemp', 'salinity' and 'surface_pressure'
to the per-divecomputer data structure.  They are filled in by the dive
computer, and normally not edited.

NOTE! All actual *use* of this data was then changed from dive->field to
dive->dc.field programmatically with a shell-script and sed, and the
result then edited for details.  So while the XML save and restore code
has been updated, all the displaying etc will currently always just show
the first dive computer entry.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Remove XML parsing special case temperature code

We had various hacky historical artifacts in our XML parsing, partly
from our legacy of parsing integer and floating point data separately
(we used to recognize certain import format differences based on whether
the data was in a floating point or integer format).  And partly from
trying to do a good job of importing crap from other dive log software.

Anyway, that actually meant that we refused to parse negative numbers,
and we ignored temperatures of zero because some diving log would do
that for missing values.

Both of these actually bit us when parsing our native XML.  Of course,
only crazy ice divers would ever notice.

Noticed by Henrik Brautaset Aronsen.

Reported-acked-and-tested-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix temperature rounding issues

Temperatures can actually be negative, which means that rounding by
adding 0.5 and casting to 'int' is not correct.

We could use '(int)(rint(val))' instead, but the only place we care
about might as well just print out the floating point representation
with a precision of two digits instead.  So if you have a dive computer
that gives you the precision, you might see '3.5˚C' as the temperature.

Remove the helper functions that nobody uses and that get the rounding
wrong anyway.

Reported-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>