Stores a user-defined element-wise function under the given name so that it
can be retrieved by get_eltwise() and invoked during query evaluation.
Examples
register_eltwise("Clamp01", function(x, ...) pmin(pmax(x, 0), 1),
overwrite = TRUE)
registered_eltwise()
#> [1] "Abs" "Clamp" "Clamp01" "Convert" "Exp"
#> [6] "Fraction" "Log" "Round" "Significant" "Sqrt"