Change values of dpdata.LabeledSystem() and multi frames for ‘abacus/scf’

[AroundPeking]

Discussed in #454

^{Originally posted by AroundPeking April 20, 2023}
Two questions I wanna ask:

1. I wanna change the values of dpdata.LabeledSystem(), like that dsys_3['virials']=dsys_2['virials']-dsys_1['virials'],
   and issue showed:
   
   So I wanna ask how to change the values conveniently?
2. Now I have much more output of SCF calculation with "abacus/scf" format, and I wanna use
   dsys = dpdata.LabeledSystem(dir,fmt="abacus/scf") to load and then transform them to deepmd format for training. But it seems that dpdata.LabeledSystem(dir,fmt="abacus/scf") do not support multi frames.
   How can I load them conveniently?