[test] Clarify and cover orphaned functions distance() and time_diffs() in data_utils.py

[adity1raut]

Description:

• Both distance() and time_diffs() (lines 23–71) carry an internal # TODO: Where is this function used? comment. They have no tests, so any accidental breakage (e.g., removal, signature change) would be silent.

Acceptance criteria

• Add unit tests with known inputs/outputs (e.g., a straight 2-point route along a meridian where distance is analytically known)
• If the functions are confirmed unused, open a separate issue to remove them — but until then, tests act as a guard

[RitikaSen27]

Hi! I’ve implemented unit tests for the distance() and time_diffs() functions using a simple analytical route along a meridian where the geodesic distance is known.

While working on this, the commits were pushed to my existing PR that implements the CSV export feature, so the tests are currently included there as well.

PR: #173

If preferred, I can separate the tests into a dedicated PR for this issue.