Column scalars

[AstroPatty]

This PR implements scalars in column arithmetic, stuff like

scaled_mass = oc.col("fof_halo_mass") / oc.col("fof_halo_mass").mean()

Scalars can also be selected, as long as they are the only thing selected. This works with the "global" mode introduced in 1.3, allowing these to be done across all ranks.

column = oc.col("fof_halo_mass")
mass_range = ds.select(min_mass = column.min(), max_mass = column.max(), mode="global").get_data()