Add convenience function for column metadata access

The data structure is inconvenient for accessing the metadata object for a given column given its name. Here's a simple accessor function for column metadata.

function getColumnMetadata(dataset, columnName){
  return dataset.metadata.columns.filter(function (column){
    return column.name === columnName;
  })[0];
}

This should be added to chiasm-dataset, and removed from:

[X] https://github.com/curran/data-reduction/blob/master/test.js#L6
[X] https://github.com/chiasm-project/chiasm-charts/blob/master/src/components/barChart.js#L7
[X] https://github.com/chiasm-project/chiasm-crossfilter/blob/master/index.js#L102
[X] https://github.com/chiasm-project/chiasm-crossfilter/blob/master/test/test.js#L117

chiasm-project / chiasm-dataset

Add convenience function for column metadata access #5