Follow Julia 1.7 broadcasting rules

[bkamins]

see JuliaLang/julia#39859

We have to decide what to do with:

julia> df = DataFrame(a=1:3)
3×1 DataFrame
 Row │ a     
     │ Int64 
─────┼───────
   1 │     1
   2 │     2
   3 │     3

julia> x = zeros(3)
3-element Vector{Float64}:
 0.0
 0.0
 0.0

julia> x .= df
3-element Vector{Float64}:
 1.0
 2.0
 3.0

julia> df[!, 1] .= df
ERROR: DimensionMismatch("cannot broadcast a data frame into 1 dimensions")

[bkamins]

@nalimilan - do you think we should only fix the tests or allow df[!, 1] .= df if df has one column?

[nalimilan]

I'd keep throwing an error. I don't see a strong reason this pattern would be useful, and as you noted at JuliaLang/julia#39859 we are a bit stricter than arrays since data frames are always 2D.

[bkamins]

OK - so I will just fix tests.