changing the scp logic and the ssh file checking logic to use a different command if no server name is set

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.

1) There is now a python alternative to the R-based wheat-source-
generation, called source_gen. The functionality to call source_gen
has been added (see ProcessorSurveys.pyL939 and L1032-1040), but not
yet switched on while we await testing of
output at the Met Office.

2) ProcessorComponents was huge (2100 lines!) and the structure was
not apparent. Each component's supporting functions are now in a
separate directory. There is also a ProcessorServer.py relating to
file down/up-loads. ProcessorComponents is now essentially a directory
 to import all the functions. This might inspire a more systematic
 (object-oriented?) structure for each component within coordinator
 in the future.