Dataframe views

We should define some kind of view into a dataframe:

# Column view

A column view should reference another dataframe and some or all of its columns. Possibly not in the same order as the original dataframe.

A more advanced version is a column view able to reference columns from different dataframes.

# Row view

A row view should hold a reference to another dataframe and be able to access (a subset of) its rows.

# Mixed row column view

Should combine the two view described above.

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As all of these are view, changes to values in a view should be reflected in the dataframes being view into.

A view should also be able to materialize itself into a non-view dataframe copying(/moving for columns?) the looked upon rows/column.