petascope - use downscaled collections in scale() operator of WCPS

[Bang Pham Huu]

It is good to optimize WCPS by using downscaled collections (if any) as WMS, example:

scale($c[ansi:"CRS:1"(0), Lat:"CRS:1"(0:100), Long:"CRS:1"(0:100)], {Lat:"CRS:1"(0:50), Long:"CRS:1"(0:50)})

step1: coverage identifier handler, $c becomes a 
Coverage object, rasql = c

step 2: subset handler, result is Coverage object 
with subset applied, rasql = c[0, 0:100,0:100]

step 3: scale handler, result is Coverage object with scale
applied, current implementation rasql = scale(c[0,0:100,0:100], [0:50,0:50])

we want that in step 3 rasql = scale(c_2[0,0:50,0:50], [0:50,0:50])

Generally, it needs 2 steps:

step 1: find the scale factor to replace with, 
by looking the grid domain of the first argument and the 2nd argument.

step2: replace the coverage id with the coverage id 
of the chosen scale level, and also adjust 
the subsets applied on that coverageId.