Fix JSON serialization, flexible parameters, and lines

[alihamdan]

• Fixes in JSON serialization:
  • 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 TypeError in Line.max_current
• Fix use of FlexibleParameters from roseau-load-flow instead of the single-phase equivalent
• Disallow single-phase transformers. Only single-phase equivalent of three-phase transformers are allowed for balanced three-phase networks
• Improve the switch checks messages to print the actual reason of the failure.

[Saelyos]

Thanks for the fixes, looks good to me