Don't depend on the current working directory

[qstokkink]

Extracted from #473

The current working directory is mutable during execution on Python code. Even though it works, this is very fragile design. There should be no need to ever depend on the cwd, given all the folders Gumby uses are in the os.environ.

We should go over all StatisticsParser subclasses to make sure they point to the stable directories of os.environ.