CRA-598 Do not write background model in depth when time-to-depth mapping
does not exist. This is applicable when we use the velocity from seismic
inversion to make the mapping. In these cases, the background model will
currently not be written to disk. This is unfortunate.
Merge pull request #42 from pdahle/master

CRA-598 Do not write background model in depth when time-to-depth mappin...