Add native support for geometry attribute

[loreabad6]

In loreabad6/post the function as_post_array() takes care of the data cube creation, but probably in a redundant way of what is offered by st_as_stars.data.frame(). Supporting this directly with stars, this PR passes the geometries as an array instead of as a list which was the previous behaviour.

library(stars)
date = seq(
    from = as.Date("2020-10-01"),
    to = as.Date("2020-10-05"),
    length.out = 5
)
geom = st_sample(st_bbox(c(xmin = 0, xmax = 1, ymin = 0, ymax = 1)), 25)
df = st_sf(id = rep(letters[1:5], each = 5), date = date, geom)
st_as_stars(df, dims = c("id", "date"))
#> stars object with 2 dimensions and 1 attribute
#> attribute(s):
#>      geom    
#>  POINT  :25  
#>  epsg:NA: 0  
#> dimension(s):
#>      from to     offset  delta refsys  values
#> id      1  5         NA     NA     NA a,...,e
#> date    1  5 2020-10-01 1 days   Date    NULL

[edzer]

Thanks!