Extend GroupBy API of DataFrame with ability to iterate groups and rows

[asmirnov82]

System information

• OS version/distro: Any
• .NET Version (eg., dotnet --info): Any

Issue

• What did you do?

Used GroupBy method of dataframe

• What happened?

Got GroupBy object. GroupBy object doesn't provide any easy way to iterate the groups and rows inside the group

• What did you expect?

I expected ability to have API similar to GroupBy operation in LINQ with ability to iterate groups and DataFrameRows inside th group (having IEnumerable<IGrouping<out TKey, out TElement>> interface)