code: function getColumnsByType(columns) {
    var columnsByType = {};
    for (var varType in VarType) {
      if (VarType.hasOwnProperty(varType)) {
        var v = VarType[varType]; // we don't want the keys to be LAT, LON, ..., but 0, 1, ...
        columnsByType[v] = [];
      }
    }
    for (var i = 0; i < columns.length; i++) {
      var column = columns[i];
      columnsByType[column.type].push(column);
    }
    return columnsByType;
  }
docs: Given columns, returns columnsByType, which is an object whose keys are elements of VarType,
  and whose values are arrays of TableColumn objects of that type.
  All types are present (eg. structure.columnsByType[VarType.ALT] always exists), possibly [].
  @private