View a subset of the rows of a dataset

A RowSubsetView is concerned only with the rows of a dataset. The view created contains the rows of the the original dataset with the keys provided in the keyList. The view maintains the order of te keyList and allows for duplicate keys. If non-column keys exist in the keylist, a KeyError() is raised.

A RowSubsetView is a subclass of the SubsetView.

__init__ (
        self,
        dataset,
        rowList,
        )