function Project(params) {
    Transform.call(this, null, params);
  }
Performs a relational projection, copying selected fields from source
tuples to a new set of derived tuples.
@constructor
@param {object} params - The parameters for this operator.
@param {Array<function(object): *} params.fields - The fields to project,
  as an array of field accessors. If unspecified, all fields will be
  copied with names unchanged.
@param {Array<string>} [params.as] - Output field names for each projected
  field. Any unspecified fields will use the field name provided by
  the field accessor.