Add GrDVI, LCN, MSAVI2

[j-miszczyszyn]

I decided to add several indicators but, despite reading the guide, I did not know how to proceed when adding more than one indicator. So I created a new branch and each separate commit is a new indicator.
I hope it works, I really appreciate the repository.
I work in R and admit that I had a problem running the test because i'm begginer in python.

I send warm regards,
Jakub

[davemlz]

Don't worry, please follow this:

• Update your branch, as it is outdated now. You can do this in the page of your branch: "Sync Branch".
• Multiple indices in one PR are okay!
• Delete LAI as LAI is not an spectral index.
• The GDVI paper you mention (Tucker et al., 1979) doesn't mention the formula you propose. Please update the reference of this index. Additionally, try a different key for this index, it shouldn't be too long, e.g. "GrDVI".
• The MSAVI2 paper you mention (Tucker, 1979) doesn't mention the formula you propose. Please update the reference of this index.
• The LCN paper you mention (Boegh et al., 2002) doesn't mention the formula you propose. Please update the reference of this index.

If you are having issues with doing the PR, remember that you can also submit the request as an Issue ;)

Cheers!