Check chirp transfers on the worker.

Chirp seems to sometimes exit with a code of 0, although the output file is
missing later.  Ensure therefore that files transfer correctly.  Relates
to #47.
Rewrite python job run script.

Should prepare us for adding more features.  Move more stuff to functions,
including setting exit codes on failure.
Change job stageout behavior to fail on missing files.
Add docstrings to job python script.
Add support for pro- and epilogue.

Basis for part of #54.
Calculate checksums on the worker.

Part of #54.
Fail job if chirp stage-in fails.
Move report merging to the worker.

Fixes #54.