EN_stability_check use of pressures

[s-good]

The EN_stability_check code requires pressure data, which it accesses from the profile data using the p() function, which in turn gets the data from depth-dependent primary variable code 25 in the WOD ASCII data. This is not always available, though, and that means the check cannot run. If there are no pressure data available, the code should attempt to convert them from the depth data. This will allow the test to run on more profiles than it can at present.