Add Preprocessors and Postprocessors to Unit Solution Procedure

[axtimhaus]

In a similar way like current rotator or the profile translation in #251 but configurable per unit class. Also would help for bulging.

They shall be run one time per unit solve before and after the actual fixed-point iteration to prepare the given incoming profile for solution and modifies the outgoing for output.

Stored as factory functions in lists per unit class (class attribute)