Added LG21 test case and its expected csv file

[adedralin]

Description

• Implemented test case LG21 with description: "should have hourly points for middle readings of 15 + 20 minute for a 60 day period and quantity units & kWh as MJ"
• Uploaded its appropriate CSV expected values file.

Co-authored by: @ChelsyLan, and Gyugang Lee

Partly Addresses #962

Type of change

• [ ] Note merging this changes the database configuration.
• [x] This change requires a documentation update

Checklist

(Note what you have done by placing an "x" instead of the space in the [ ] so it becomes [x]. It is hoped you do all of them.)

• [x] I have followed the OED pull request ideas
• [x] I have removed text in ( ) from the issue request
• [x] You acknowledge that every person contributing to this work has signed the OED Contributing License Agreement and each author is listed in the Description section.

[codingfiddler]

Also co-authored by @Username13555

[huss]

Thanks to @adedralin, @ChelsyLan, @Username13555 & Gyugang Lee (I hope I have the names correct) for this contribution. I don't see a record that any of you signed the CLA. Assuming this is correct, please use the link in the description/Checklist to do this. If you think my records are wrong then please let me know.