Don't display materialisation info by default

[hadley]

e.g. in the readme:

out$mean_bill_area
#> materializing:
#> 
#> [1] 770.2627 656.8523 694.9360 819.7503 984.2279

I'd suggest something like:

out$mean_bill_area
#> materializing query: see last_materialization() for details
#> [1] 770.2627 656.8523 694.9360 819.7503 984.2279

last_materialization()
#> ---------------------
#> --- Relation Tree ---
#> ---------------------
#> Projection [species as species, sex as sex, mean_bill_area as mean_bill_area]
#>   Order [___row_number ASC]
#>     Filter [!=(species, 'Gentoo')]
#>       Projection [species as species, sex as sex, mean_bill_area as mean_bill_area, row_number() OVER () as ___row_number]
#>         Projection [species as species, sex as sex, mean_bill_area as mean_bill_area]
#>           Order [___row_number ASC]
#>             Aggregate [species, sex, min(___row_number), mean(bill_area)]
#>               Projection [species as species, island as island, bill_length_mm as bill_length_mm, bill_depth_mm as bill_depth_mm, flipper_length_mm as flipper_length_mm, body_mass_g as body_mass_g, sex as sex, "year" as year, bill_area as bill_area, row_number() OVER () as ___row_number]
#>                 Projection [species as species, island as island, bill_length_mm as bill_length_mm, bill_depth_mm as bill_depth_mm, flipper_length_mm as flipper_length_mm, body_mass_g as body_mass_g, sex as sex, "year" as year, *(bill_length_mm, bill_depth_mm) as bill_area]
#>                   r_dataframe_scan(0xdeadbeef)
#> 
#> ---------------------
#> -- Result Columns  --
#> ---------------------
#> - species (VARCHAR)
#> - sex (VARCHAR)
#> - mean_bill_area (DOUBLE)

And maybe have some option to suppress the notice about materialization altogether?

[hannes]

opt in only please