Fix bugs in JSON serialization

[alihamdan]

Found and fixed the following bugs:

• Fix version checks in network_from_dict
• Emit proper warning for older versions
• Add missing max_loading for lines
• Add missing max_loading for transformers
• Add missing tap for transformers
• Fix error due to sorting of line parameters and transformer parameters with heterogeneous ID types
• Fix conversion of single and center transformer types to proper vector groups
• Fix upgrading of max_current, conductor_type, and insulator_type of line parameters

These bugs were found with testing more comprehensive JSON files. I added files for all JSON versions and the corresponding file generator scripts.

I also added a test to ensure these files do not get modified by mistake when refactoring the code base.