account for topography in integrate_to_ppi() also for sea level profiling

[adokter]

Currently no digital elevation map information is used when the height reference for integrate_to_ppi() is sea (only for ground). However, as a result the method may interpolate densities into altitude bands occupied by mountain ranges.

Note to self: we accounted for this in the WGF project, need to include this into integrate_to_ppi