setup () {
    let width = this.container.getBoundingClientRect().width;
    let colWidth = this.colWidth();
    let numCols = Math.floor(width/colWidth) || 1;
    let cols = [];

    if (this.maxColumns && numCols > this.maxColumns) {
      numCols = this.maxColumns;
    }

    for (let i = 0; i < numCols; i++) {
      cols[i] = {height: 0, index: i};
    }

    let wSpace = width - numCols * colWidth + this.gutter;

    return {cols, wSpace};
  }
Initializes an array of empty columns
and calculates the leftover whitespace.

@return {{cols: Array, wSpace: number}}
@private