Consider making TDAmapper citable

[hwaldstein]

As of 2014, GitHub allows projects to get a DOI (per commit) to use for citation in academic works. Because of the applicability of this software to academic purposes, setting up TDAmapper to have a DOI may be a good thing to consider. The guide for the process of getting a DOI can be found here: https://guides.github.com/activities/citable-code/.