Add readAll() for SpatRaster and SpatRasterDataset

[dfriend21]

This pull request adds a readAll function for both SpatRaster and SpatRasterDataset that reads all values into memory. set.values() already accomplishes this for a SpatRaster. However, there isn't an equivalent function for a SpatRasterDataset, so I created a function to do this.

It didn't make sense to create another S4 version of set.values() for SpatRasterDataset since the function would only be for reading values into memory, not for setting values. Because of this, I created a readAll function, and since I was making one for SpatRasterDataset I also made one for SpatRaster. Part of the reason for this is that I feel like it'd be nice to have a dedicated function for reading in the values whose name more clearly indicates it's purpose, since it isn't obvious from the name set.values that the function reads in values from memory (related: #507).

Here's an example - it's also a good example of my usual use-case for explicitly reading values into memory.

library(terra)

# create the URLs for reading in the rasters from the WCS
layers <- c("litter", "shrub")
years <- c(1985, 2023)
template <- c("WCS:https://www.mrlc.gov/geoserver/rcmap_%s/wcs?coverage=rcmap_%s__rcmap_%s_%s")
paths <- lapply(layers, function(layer_i) {
  sprintf(template, layer_i, layer_i, layer_i, years)
})

# define our area of interest
e <- ext(-1631650, -1626603, 1514452, 1517650)

# read in a single layer and read values in with 'readAll'
r <- rast(paths[[1]][1], win = e)
inMemory(r)
readAll(r)
inMemory(r)

# read in the all the layers and convert to a SpatRasterDataset
lst <- lapply(paths, rast, win = e)
sds <- sds(lst)

# use readAll to read all values into memory
sapply(sds, inMemory)
readAll(sds)
sapply(sds, inMemory)

plot(rast(sds))

[rhijmans]

Thank you, that looks good. I think I wanted to hide this kind of method to avoid malpractice, but I am sure there are good use-cases in which readAll could speed up the analysis.

[rhijmans]

I renamed terra::readAll to to terra::toMemory because raster::readAll uses "object" in stead of "x" so that gave a conflict that is a bit tricky to resolve; and to better separate it from readStart/Values/Stop.

I changed the methods so that they do not change the objects in-place (and added set.values<SpatRasterDataset> for if you want to do that. Only set.xxx should change objects in place (the problem is that all possible copies of the object are also changed).