Research if we can use custom start and end values for grouped pad

[EdwinTh]

When doing grouped padding we can only supply a universal start and end value. When you want to give each group a custom value you have to do some wrapper programming yourself, like doing loops or split and apply. It is would be reasonable to supply this functionality within the function.

See https://stackoverflow.com/questions/69726957/fill-data-gaps-with-pad-using-group-returns-error/69798556#69798556

and related issue #85