[FEATURE] make tap_pos optional

[petersalemink95]

For users that want to use the automatic tap changer power flow, it does not really matter what the original tap_pos is. They are just interested in the final tap_pos, found by the automatic tap algorithm. At this moment tap_pos is required, since it is necessary for the other algorithms.

We can make tap_pos optional, if we assign it the value of tap_nom when it is not given

[mgovers]

current functionality clamps the tap position: https://github.com/PowerGridModel/power-grid-model/blob/09ee5a6e3a65d21cb2d0dbc8bc18b29ecc405c5f/power_grid_model_c/power_grid_model/include/power_grid_model/component/transformer.hpp#L145-L149 (and idem for 3w-transformer).

this existing behaviour may impact users. if we decide that existing functionality is good enough, documentation should be improved