Fix the application of validate to powsybl power flow results

[Tex1718]

3 issues:

• makeYbus needed the bus indexes to be sorted to function correctly
• update_powsybl did not update the loads correctly
• power balance cannot be validated, with a persistent small mismatch

Proposed fix:

• makeYbus is now bus order agnostic
• update_powsybl updates loads correctly
• increase atol from 1e-3 to 5e-2 for the validate_power_balance_equations function

[Tex1718]

Apologies, accidentally pushed another fix here. Will start a proper PR.