Additional vegetation spectral indices of interest

[HaleySchuhl]

Is your feature request related to a problem? Please describe. Are there additional vegetation indices supported here that we could add to plantcv's spectral_index subpackage?

Supported Indices not already in pcv

        "GNDVI": "(msi.nir - msi.green) / (msi.nir + msi.green)",
        "ENDVI": "(msi.nir + msi.green - 2*msi.blue) / (msi.nir + msi.green + 2*msi.blue)",
        "NNIR": "msi.nir / (msi.nir + (msi.rededge + msi.green))",
        "NGRDI": "((msi.green) - (msi.red)) / ((msi.green) + (msi.red))",
        "NLI": "((msi.nir**2) - msi.red) / ((msi.nir**2) + msi.red)",
        "DVI": "msi.nir - msi.red",
        "RDVI": "(msi.nir - msi.red) / ((msi.nir + msi.red)**(1/2))",
        "MDD": "(msi.nir - msi.rededge) - (msi.rededge - msi.green)",
        "HUE": "np.arctan((2 * (msi.red - msi.green - msi.blue)) / (30.5 * (msi.green - msi.blue)))"```

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