Fix WSClean exception writing temporary file

In many of the CWL steps that invoke WSClean, the argument to -temp-dir was given as user-specified input (a path pointing to the scratch directory used by Rapthor). Problem is that this directory is not mounted inside a Docker container, hence cannot be written to.

This fix uses the $(runtime.tmpdir) CWL variable to determine the location of the temporary directory at runtime. Its value can be managed by the CWLTool/Toil command-line option --tmpdir-prefix. Rapthor sets this option based on the value of dir_local in the parset file.

Closes RAP-145