Spidercrab - integrity testing

Implemented integrity testing in 84299d4 (available as a separate script, executed on the beginning of every spidercrab test).

New useful functionality within don_utils.py - `get_ocean_root_dir()`.
Ocean root directory is now determined by "indexing" file `OCEAN_ROOT/.__ocean_root__`, where `OCEAN_ROOT` is an absolute path to your repository directory :)

I recommend refactoring every change-working-directory-related and system-path-append-related tricks to use above funtionality.