Add a `@bycol` macro-flag

[pdeffebach]

Can't believe this doesn't exist. I would really like it.

@MatthewRGonzalez do you want to make a PR?

[MatthewRGonzalez]

Sure-- I'll check this out @pdeffebach

[pdeffebach]

@MatthewRGonzalez I've marked this as 1.0.

I want to get a 1.0 release together soon. People are going to get mad if we do too many v0.x.0 releases. But I think this is a feature that should go into 1.0

[MatthewRGonzalez]

@pdeffebach Do you have any examples in mind for applications of the @bycol flag? Are you looking for something like:

@chain df begin
           transform(@bycol r"x" .=> mean)
       end

as opposed to

@chain df begin
           transform(Cols(r"x") .=> mean)
       end

[pdeffebach]

No, @bycol is the opposite of @byrow.

@transform df @bycol :y = f(:x)

gets lowered to

transform(df, :x => f => :y)

that is, it's redundant by itself. But if you have a block it matters

@rtransform df begin
    :z = g(:a)
    @bycol :y = f(:x)
end

gets lowered to

transform(df, 
    :a => ByRow(g) => :z,
    :x => f => :y)

[MatthewRGonzalez]

Got it-- I'll start working!