add "materialize" method to dataframe to force evaluation and return a new materialized dataframe object

[sunliren2005]

We found performance issue when we drive more dataframe from a common source dataframe. The iterator is repeating on the common part. we need something like, so it won't start from beginning.

function materialize(){ 
   return new dataForge.DataFrame({
            values: this.toArray()
})};

[sunliren2005]

I found bake. Sorry.