Update consts.py with merra2

[AUdaltsova]

Pull Request

Description

Added consts to include merra2 std and mean. India is the default, Britain values added separately

Fixes #

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [X] Yes

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

• [X] Yes

Checklist:

• [X] My code follows OCF's coding style guidelines
• [X] I have performed a self-review of my own code
• [X] I have made corresponding changes to the documentation
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] I have checked my code and corrected any misspellings