Create icesat2_dataset_key_variables.yml

[andypbarrett]

This creates a YAML containing key variables for Level 2A and Level 3A products.

I am adding key variables for ATL: 03, 04, 06, 07, 08, 09, 10, 11, 12, 13.

I am using a YAML to make this machine and human readable. Yaml structure is

products:

• dateset: dataset id
  name: long name for dataset/product
  variable: one or more key variables as a list
• dataset:
  .
  .

[andypbarrett]

I will try to add key variables for most products by the end of the day.

At the moment, I am only adding single variables but this ignores coordinate and quality variables. I can add these as well if necessary.