Merge branch 'missing_dependency' into 'master'

Fixed missing dependency between init and density in the case of sub-tasks.

Not sure whether these dependencies disappeared at some stage or whether they have never been there but they are clearly missing.

That fixes #153 and potentially the MPI issues if they are due to missing links. Can't really test it from here. 
@pdraper that might be of interest to you.  

See merge request !151