samtools_depth should map over datasets

[ammaraziz]

The samtools_depth treats a dataset collection as many bams. I think this not really standard usage of samtools depth which is (I think) performed on individual bams separately. This wouldnt be a big problem if the item names in the collection are transferred over into the output depth file. However this is not possible as samtools depth does not output header information. The end result is a not useful output.

The solution is allow the user to select single or multiple bams, similar to how samtools_coverage handles this situation.

[bernt-matthias]

Workaround is to restructure the input collection, ie to a nested collection, where the inner collections contain a single element.