Triage CLI should support relative paths

```
triage experiment experiment_config/simple_test_skeleton.yaml --project-path 'output' --validate-only
/usr/local/lib/python3.6/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: .
""")
[FileNotFoundError] [Errno 2] No such file or directory: ''
```

In order to work you should run:

```
triage experiment experiment_config/simple_test_skeleton.yaml --project-path '/triage/output' --validate-only
```
```

Start with the `triage experiment` command and its `--project-path` handling. Reproduce the shown `--validate-only` command using a relative path, compare it with the absolute-path example, and verify that relative paths work without a FileNotFoundError.

Written by the indexing model from the issue text.