Generify for other schools: Generify / Encapsulate implicit data assumptions of replacer class

[ArchibaldBienetre]

With the current implementation, while the rest of the code is quite generic,
org.example.csv2tex.placeholders.PlaceholderReplacerImpl makes strong assumptions about what values there are in the data.

Many of these are likely specific to the Erfurt school, e.g. levels "rot", "grün" or grades 1, 2, 3, 4 (where other German schools will go for 1, 2, 3, 4, 5, 6 and other countries have their own thing).

If we have other interested parties, we should generify this code.

[ArchibaldBienetre]

Sick with CoViD as I am, I accidentally pushed this directly to the main branch 😕 🤒 🤦‍♂️
4a271fa

I may just leave it in, if the tests run through

[ArchibaldBienetre]

Fixed.

FYI @DonMischo , I'll no longer label your changes a hack, much cleaner now 🙂