Make Preprocessor more generic

- allow for extension in regard to additional functions
- flexible / extendable unit dict
- optional: better / more complete default unit dict covering all schema cases (maybe redesign it so that the output is the key and the potential input values are in a list)
- flexible / extendable field array

Start by locating the Preprocessor implementation and inspecting how its functions, unit dict, and field array are currently defined. Clarify the intended extension points and schema coverage before changing the design. Done means callers can add functions, units, and fields without modifying the core defaults, with the optional default-dictionary redesign agreed and covered by existing project checks.

Written by the indexing model from the issue text.