Fixed empty submodule problem

• Included a zip file that contains the submodule broadband-echo-stats code (only the files needed to plot figure_23).
• Updated README to include info for directly downloading code from repo broadband-echo-stats as zip file if users choose to download the GitHub zip file and not Zenodo (which by default does not include the submodule code).

Small updates to first release after publication

Small updates to include publication DOI and citation, also corrected filename for plotting appendix figures

First release

First release, identical to the copy submitted with the manuscript.