Anchors for tiled datasets

When we have multiple separate raster files making up a mosaic, like SRTM, we want all of their ranges to be anchored at the identical floating point position. With typed datasets from RasterDataSources we can just explicitly include the anchor in the RDS extension. Then all files from the dataset will load on a bit-identical grid. If we dont know the anchor for a dataset, we can use the first file, and shift all the others within a tolerance. If they really need it, users could do `Raster(filename; anchors=anchors(otherraster))` which returns `(X=xanchor, Y=yanchor, Ti=nothing)` etc. And errors if the anchor is just wrong for that file - with a tweakble tolerance. See #1075