"DataFrame.select" operation for selecting rows

[andrus]

In addition to DataFrame.selectColumns add a new DataFrame.select(..). This will allow to sample DataFrame contents, implement more window functions, etc. Selector should support filtering, reordering and duplicating rows.

df1 = df.select(1, 5, 3 );
df2 = df.select(asList(1, 5, 3));

Series<Integer> s = ..
df2 = df.select(s);