Refactor fix_pressure_levels function

[atteggiani]

Overview

In the current implementation, the fix_pressure_levels function doesn't return anything (i.e. returns None) when there is no pressure coordinate:

um2nc-standalone/src/um2nc/um2netcdf.py

Lines 1002 to 1005 in b44e549

	try:
	pressure = cube.coord("pressure")
	except iris.exceptions.CoordinateNotFoundError:
	return

and within the process_cubes function the fix_pressure_levels function is used as it follows:

um2nc-standalone/src/um2nc/um2netcdf.py

Line 586 in b44e549

c = fix_pressure_levels(c) or c # NB: use new cube if pressure points are modified

Improvement

If the pressure coordinate is not found, the fix_pressure_levels should return the cube as it is, so in the process_cubes we can simplify the call within process_cubes.

A suggestion for the changes of the fix_pressure_levels and of the process function is shown below:

fix_pressure_levels function

try: 
     pressure = cube.coord("pressure") 
 except iris.exceptions.CoordinateNotFoundError: 
-     return 
+     return cube

process function

- c = fix_pressure_levels(c) or c 
+ c = fix_pressure_levels(c)