Added tests for Greenplum

Added a script `test/test_gp.sh` that run deepdive tests under a GP settings. When running `test.sh`, If `gpfdist` command is present, the script will be executed.

Now we run following tests for GP:
- unit tests (without parallel grounding)
- biased coin (with / without parallel grounding)
- spouse example (with / without parallel grounding)

For chunking, there are some file permission issues that I haven't been able to resolve yet. (so both tests with / without parallel grounding fails)

If it's OK to skip chunking example in the GP test, we can set up the automatic test pretty soon.

I thought it would be a bit safer to do this... e.g. if groundingPath is "", it could be a bit dangerous?