remove-background WDL: workflow can fail if some outputs are not created

[sjfleming]

If there is any error that prevents an output file from being created, for example the output PDF, this is handled gracefully by cellbender. However, it is not handled gracefully by WDL, which only allows "required" outputs. If one of these outputs does not exist, the workflow fails.

Proposed solution:
In the WDL, ensure each file exists before the task ends. This might amount to just a couple of bash

touch filename

commands.

[sjfleming]

Also need to handle trailing slashes for the input called output_directory, because the current behavior is to insert an extra trailing slash, regardless of whether one already exists. This leads to unexpected behavior.

[sjfleming]

Closed by #238