[FEATURE] Option to return values/variance with forcefully padded flow bins

[andrzejnovak]

For further work (eg plotting) it would be useful to have the option to return shapes as if all flow bins existed, padded with 0s. This could be sth like values(allflow=True) or values(forceflow=True). This would allow the same flow access as uproot TH1s making the interface a bit more uniform. Currently one has to run a series of checks on the dimensionality of values(flow=True) or hasattrs and checks on underflow/overflow traits in each axis.