Push and document ERA5 data download and processing files

[FanWangEcon]

To prepare for packaging up and functionalizing existing data input, we want to organize files for downloading and processing ERA5 data, and push code to repository.

Push download downloading and processing code to the Repository. To the Sandbox folder. Feel free to create additional subfolders as needed.

1. Push key files for downloading ERA5 files.
   • Push file, link to issues
   • Provide simple documentation either in README or in file what the file does, highlight input and output
2. Push key files for process (averaging) ERA5 files, hourly or dayly aggregations, and ways to select
   • Push file, link to issues
   • Provide simple documentation either in README or in file what the file does, highlight input and output

[FanWangEcon]

@szkaifeng

Please provide basic information, at the top of each file and inside code blocks, on what:

• utci_convert_loop_1990_2000.R and
• utci_convert_loop_2010_2020.R

are separately trying to achieve.

We generally want to document at three spots:

1. For files in folder, document in README.md for folder, very briefly what each file does
2. At the top of each file, what the file's objective is
3. At the start of each code block/segment, what the next block of code does.

Documentation is as important as generating the code itself.