add modpolish mode from homopolish

[pmenzel]

https://github.com/ythuang0522/homopolish#quick-usage-for-polishing-modification-errors

however, there is an issue with installing homopolish v0.4.1 via bioconda.

when using https://github.com/ythuang0522/homopolish/blob/master/environment.yml (+ homopolish itself) via conda, it fails on libffi.so.6, which might not be available in the base system, and would require libffi=3.3 from conda, but homopolish requires >=3.4.