Generic multispectral index runnable

[paulmis]

Add a generic multispectral index runnable that computes an arbitrary multispectral index according to its formula:

• [x] Create a base multispectral index runnable class
• [x] Provide a way to initialize required spectra
• [x] Annotate the index with a generic description
• [x] Turn NDVI into a multispectral index runnable