Adjust vol to accept filenames (fixes #3701)

[fabaff]

Description:
The data: needs to allow a filename and not a path.

Related issue (if applicable): fixes #3701

Example entry for configuration.yaml (if applicable):

sensor:
  platform: gtfs
  origin: STOP_ID
  destination: STOP_ID
  data: DATA_SOURCE

Checklist:

If code communicates with devices, web services, or a:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @robbiet480 and @imrehg to be potential reviewers.