[FEATURE] Continuous / decimal Automatic tap changer

[nitbharambe]

Some users wish to have continuous tap positions instead of discrete. We can avoid any undiscovered max iteration failure edge case of outer loop tap changer. Performance would remain in similar scale as a single iteration.

Changes / additions in API

• Add a new TapChangingstrategy.continuous_tap to trigger the NRPF tap position calculation.
• Add a new attribute of tap_pos_decimal in transformer_tap_regulator output.
• We do not allow input of decimal tap_pos for non automatic_tap_changer calculations.
• Only possible for Newton raphson power flow. Other calculation methods need to raise error.

Implementation

• The extra argument gets passed along to the solver directly.
• Modify the Jacobian / unknowns to include the tap position as variable in the block element of NRPF.
• Derive corresponding Jacobian equations. We already did this derivation in past for symmetric calculations. Double check if same can be applied to asymmetric trivially.
• Pass in tap_pos unknown in the output calculation. This can be a bit tricky as we derive all results using voltage vector.