Petascope - Extend Coverage/Collection Iterator in WMS styles

[Bang Pham Huu]

Currently, WMS style (created by WCPS/Rasql fragment) only supports 1 coverage/collection iterator with reserved keywords
$Iterator for Rasql and $c for WCPS which references to the layer which defines the WMS style. Example:

$c + 20 or $Iteartor.1 - $Iterator.2 (.1 and .2 are bands)

However, if one imported coverages separately (e.g: separate Sentinel 2 bands - BigDataCube Germany NDVI is an example), he could not make an NDVI stye from 2 separated layers. Therefore, WMS style is extended to combine from multiple layers:

Allow one to add contributing coverage/collection names
directly in the layers' style, e.g:

Germany_B4 layer defines this NDVI style:
  ($Germany_B8 - $c) / ($Germany_B8 + $c)

Germany_B6 layer defines this TEST style:
  $c + $Germany_B4 + $Germany_B5

And the GetMap request will be: layers=Germany_B4,Germany_B6&styles=NDVI,TEST