partial pressures variables code cleanup

[Willem Ferguson]

Robert,

I would appreciate some advice from you.

1) The pressures structure contains a member for the partial pressure of 
each of the three possible gases (o2,n2,he). At present these members 
are defined as double.
If one could change their type to o2pressure_t (this is really an int16 
in units of mbar) then one could use constructs such as 
pressures.o2.mbar, consistent with usage throughout much of the existing 
code base. o2pressure_t has a maximum value around 65 bar, thus I do not 
think any of the pressures members will have magnitudes that exceed the 
max size of o2pressure_t. Comments, please?

2) The variable plot_info->o2pressure is really a left-over from before 
you created the pressures structure. Is there a case to be made to get 
rid of this variable, since its function has been replaced by the 
pressures structures?

Kind regards,
willem