Find Distance Calculation in source code from Chang Lab

There are two main papers that we are trying to emulate. Each of them must have used a specific distance calculation to determine the difference between two mitochondrial sequences.

Please look through the code on their github repo and find the section that performs the calculation.

Xu et al., eLife, 2019. ([Howard Chang Lab](https://changlab.stanford.edu/), Stanford) "Single-cell lineage tracing by endogenous mutations enriched in transposase accessible mitochondrial DNA." [paper](https://elifesciences.org/articles/45105), [github repo](https://github.com/ChangLab/ATAC_mito_sc)