Add citation for thinning algorithm

**Book page:** `03_stress/02_logic.ipynb`

**Error:** Not an error persay, but it states that the thinning algorithm has been "adapted from Banks et al.", but with no further information, "Banks" is insufficient for identifying the source.

**Suggested fix:** Add the year and a link - e.g. `[Banks et al. XXXX](linktosource)`