Fix issues in CCR handling

In commit 0d7c192e6edc ("For CCR dives, the diluent cylinder is the
current cylinder") a few things got broken. This tries to undo those
changes and adds expanded XML output.

1) Calculate correct partial pressure of oxygen to be plotted on
   dive profile, taking into account the oxygen sensor data.
   Currently, erroneously, OC PO2 values are shown, due to an
   erroneous calling parameter to fill_pressures().
2) Read start and end cylinder pressured correctly.  some wrong
   assignments were done in file.c. This is now corrected and the correct
   cylinder pressures are shown in the equipment tab.
3) Write correct cylinder pressures to XML. Currently the data for
   the two cylinders are written to XML the wrong way round
   (diluent pressures = oxygen and vice versa).
4) Expand XML output:
   a) Write oxygen sensor data to XML
   b) Write no_of_02sensors to XML

Signed-off-by: willem ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>