Enhanced features

1. Edited encoding function on Frame app to enable it when multiple columns selected.
2. Edited Groupby app to show To Frame option for size method.

Bug fix

1. Fixed bug on Frame app using isnull or notnull from Add, Replace function.
2. Fixed typo on Evaluation app.
3. Fixed inner function vp_create_permutation_importances() to operate for numpy objects.
4. Fixed Subset app to show .index code on condition operation for Series object.
5. Fixed Subset app for 0 named index or column.