Add an option to select columns based on eltype

[davidanthoff]

For example, select all Float64 columns.

One option might be df |> @select(<:Float64)... We should also take a look how this is done in dplyr.

[Yijia-Chen]

For the example above, should the syntax be df |> @select(::Float64)? Because <:Float64 already means a subtype of Float64, while :Float64 confounds with a column name selection.