refactor of group 8 (tests and examples) completed:

- Introduced `test_gap_detection.py` with comprehensive tests for time-grid gap detection functionalities, covering methods in `grid_builder.py` and `gp_plotting.py`.

- Added `test_gap_plots.py` to validate GP grid and plot-data preparation utilities, including tests for correlation length and data clustering.

- Added symbolic links for example data files in the tests directory: `test_gp_per_visit.yaml`, `test_lc_gp_rv.dat`, `test_rv_only.dat`, and `test_rv_only.yaml`.