[FEATURE] Support pandapower 3.5.4

[nitbharambe]

Describe the feature request

I see these extra columns have come up in new pandapower version. Fixes fails in #485

pp_net.res_gen_3ph.columns
Index(['vm_a_pu', 'va_a_degree', 'vm_b_pu', 'va_b_degree', 'vm_c_pu',
'va_c_degree', 'p_a_mw', 'q_a_mvar', 'p_b_mw', 'q_b_mvar', 'p_c_mw',
'q_c_mvar'],
dtype='object')

Towards that extent, you can use commits in #485 for other changes with tests

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[furqan463]

The gen element 3ph output structure was added in pandapower=3.5.0 in PR e2nIEE/pandapower#3010
Before this, the 3ph load flow of pandapower involving gen element was broken.
As such, In my opinion we don't need to worry about pandapower<3.5.0 as power flow involving gen element is broken there and res_gen_3ph structure is not defined.
Meaning, we can adopt this structure irrespective of pandapower version.