BUG: Do not filter surface altitude/pressure fields

Currently, surface altitude and surface pressure pp fields will be thrown away if specifying a stash constraint (unless including the reference fields stash code also).  This is a change in behaviour from before the pp load optimisation work took place which translated iris constraints to lower level pp constraints (stash only constraint).  Context: https://github.com/SciTools/iris/pull/1243

```Python
phenom = 'phenom.pp'
orography = 'orography.pp'
 
stash_constraint = iris.AttributeConstraint(STASH='m01s00i010')
cube = iris.load_cube([um_dat, orography_dat], stash_constraint)
```
The resulting cube has no surface altitude coordinate.

Workaround:
```Python
cube, _ = iris.load_cubes([um_dat, orography_dat], [stash_constraint, 'surface_altitude'])
```

The bugfix in this PR is applicable to both surface altitude and surface pressure fields.