Add vNDVI to formulas.py

vNDVI is a new RGB/Visible index derived from a genetic algorithm trained against citrus, grap, and sugarcane crop data.

The index is not normalized, so much care has to be taken when applying it as values will exceed the expected 0-1 range.

Reference paper:  
https://www.sciencedirect.com/science/article/pii/S016816991932383X

Requested by:  
Patrick Cochard

app/api/formulas.py

@@ -19,6 +19,10 @@ algos = {
         'expr': '((N + G) - (2 * B)) / ((N + G) + (2 * B))',
         'help': _('Enhanced Normalized Difference Vegetation Index is like NDVI, but uses Blue and Green bands instead of only Red to isolate plant health.')
     },
+    'vNDVI':{
+        'expr': '0.5268*((R ^ -0.1294) * (G ^ 0.3389) * (B ^ -0.3118))
+        'help': _('Visible NDVI is an un-normalized index for RGB sensors using constants derived from citrus, grape, and sugarcane crop data.")
+    },
     'VARI': {
         'expr': '(G - R) / (G + R - B)',
         'help': _('Visual Atmospheric Resistance Index shows the areas of vegetation.'),