countValue(valueToCount, columnName = this[__columns__][0]) {
        return this.filter(
            (row) => row.get(columnName) === valueToCount
        ).count();
    }
Get the count of a value into a column.
@param valueToCount The value to count into the selected column.
@param {String} [columnName=this.listColumns()[0]] The column to count the value.
@returns {Int} The number of times the selected value appears.
@example
df.countValue(5, 'column2')
df.select('column1').countValue(5)