Benchmark assembly task doesn't like it when the output directory doesn't exist

[michaelbarton]

If the directory supplied to --output doesn't exist then the biobox.yaml is
erroneous copied to this location as a file, and then subsequence copy actions
fail because it is not a directory.

[michaelbarton]

The solution is to create this directory first, before doing the copy action.

[michaelbarton]

I think adding this to the verification for this biobox type would be useful too.