aggregating multiple columns in data.table
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I have the following sample `data.table`:

    dtb <- data.table(a=sample(1:100,100), b=sample(1:100,100), id=rep(1:10,10))

I would like to aggregate all columns (a and b) by id using `colSums`, for example.  What is the correct way to do this?  The following does not work:

     dtb[,colSums, by="id"]

This is just a sample and my table has many columns so I want to avoid specifying all of them in the function name