[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

Discussing options here.
Add a new TapChangingstrategy.continuous_tap
Make input and output tap_pos to be floating instead of int

Implementation

• Only possible for Newton raphson power flow. Other calculation methods need to raise error.
• Modify the Jacobian / unknowns to include the tap position as variable in the block element. Calculate corresponding jacobian equations.