Update - frame.py

I added some examples for the following functions: product, n_chunks, is_empty, insert_at_idx, clone, get_columns, fill_null, fill_nan, with_columns, unique, groupby.groups, groupby.first, groupby.last, groupby.sum, groupby.min, groupby.max, groupby.count, groupby.mean, groupby.n_unique, groupby.quantile, groupby.median.