Add measurement(s) of scatter

The scatter of data about the ridge line is of general scientific interest. E.g., Baker et al. 2023 (https://academic.oup.com/mnras/article/518/3/4767/6843609) uses scatter to identify which relationships are fundamental and which is a consequence of the fundamental ones. Adding an estimate of scatter with one or more methods would allow for comparison with other works and further exploration of the processes which may drive this scatter.

Review the package's existing scaling-relationship fitting entry points and any notebooks or examples that analyze fitted relationships. Compare the scatter methods discussed in Baker et al. 2023 and decide which method or methods fit this package's data and API. Done means the package provides a documented scatter estimate that supports comparison with related work.

Written by the indexing model from the issue text.