Before this commit, the process_pre_job_server_download() function would
check if the necessary tar file exists on the remote server. But, because it is
 written there by sftp, and sftp doesn't generate any lock file or temp files, in-progress sftp writes look like they are available and the job continues. With this change, the job will only process if the tar file is completely written.

restoring the original sys.exit to ProcessorUtils, dealing with exit commands in the unit tests by catching them

Because of changes to EpiPrepLister in the last few months, the file listers have changed name and are too complicated to guess. Now, it will be necessary for the user to specify them in the input config file. EpiPrepLister's docstring will have to guide the user clearly enough.

This relates to developments to ease the maintenance
of the forecast system. With this development, there is no R code to
actively maintain. Moving from git project wheat-source-generation to
ews-source-generation. The python code has been validated for
identical performance to the R version. The only differences are
small changes from the underlying shapefile reading packages
(see notes-on-rewriting-in-python.txt in ews-source-generation).

Keeping the option to used the R code for some time in case any
follow-up investigation is required.