fix [ on a worldArray with other than two layers

[achubaty]

Found while investigating #49; unrelated to it and worth landing on its own.

[ on a worldArray hardcoded the layer count when reshaping the extracted values:

dim(cellValues) <- c(NROW(pCoords), 2L)

so it only ever worked on a two-layer array:

w1 <- createWorld(minPxcor = 0, maxPxcor = 4, minPycor = 0, maxPycor = 4, data = 1:25)
w2 <- createWorld(minPxcor = 0, maxPxcor = 4, minPycor = 0, maxPycor = 4, data = 26:50)
w3 <- createWorld(minPxcor = 0, maxPxcor = 4, minPycor = 0, maxPycor = 4, data = 51:75)

a3 <- stackWorlds(w1, w2, w3)
a3[1, 1]
#> Error: dims [product 2] do not match the length of object [3]

A single-layer stackWorlds(w1) failed the same way. Now uses dim(x)[3].

The sibling [,worldArray,missing,missing method already computed this correctly, which is why a3[] worked while a3[1, 1] did not — only the coordinate-indexed method was affected.

I checked the rest of the worldArray API against a three-layer array while I was in there: of(), NLset(), [<-, [[, numLayers(), layerNames(), plot() and world2spatRast() all already handled arbitrary layer counts. This was the only site.

Tests cover one, three and four layers.

Verification

• R CMD check --as-cran: 0 errors, 0 warnings, 0 notes
• devtools::test(): 0 failures

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