Address Reviewer Comments

[mengqi-z]

This pull request includes the following main updates:

1. Add "State of the Field" in the paper
2. Update the paper figure to indicate the unit of Fahrenheit based degree-days (degree-hours)
3. Add more details in the figure caption
4. Upload paper.pdf to the paper folder
5. Add Contributing section in README
6. Improve documentation for all the functions' arguments
7. Clarify raw temperature data units by adding the attribute in the output table from helios::read_ncdf
8. Update unit column in the helios::hdcd output table to indicate it is Fahrenheit-based degree-days/hours