Returns a ReadOnlyChainDaf that reads from daf but rejects
writes. Implementation delegates to chain_reader() with a single
entry; there is no separate read-only class.
Examples
d <- memory_daf(name = "inner")
add_axis(d, "cell", c("c1", "c2"))
ro <- read_only(d)
daf_name(ro)
#> [1] "inner"