Pandapower attr enum

[furqan463]

Fixes #315
as enums are only required for pandapower. json, excel and vision converters do not have raw strings in src files.

Changes proposed in this PR include:

Adds attribute enums for pandapower.

[furqan463]

Created attr enums by iterating over an empty pandapower network tables and adding all columns to a set then had to enter some manually. However, there are several attr enums that are not being used inside pgm_io. Should we retain the exhaustive enum as it is, are somehow retain only those attributes that are actually being utilized in pgm_io?.

[figueroa1395]

Hello @furqan463,

Thanks for the contribution. At the moment we are a bit occupied, so we expect to review and give feedback sometime next week.