Remove hard-wired PFT indices from cable_psm.F90

Code in `cable_psm.F90` utilises IF statements such as:

```
IF (veg%iveg(i) .LT. 16) THEN
```

These should be replaced with:
```
IF (veg%iveg(i) .LT. lakes_cable) THEN 
```

where `lakes_cable` is defined in `cable_surface_types.F90`.

Other changes in `cable_psm.F90` are coming for the GW work, wait for this work to make the change.