zellen: use er.lua for euclidean rhythms

[sarweiler]

well, i never noticed the er.lua module and implemented my own euclidean rhythm module. as er.lua does the same very elegantly in a lot less code (bucket method! beautiful!), of course i will use this, rather than my own bulky recursive implementation. sorry for duplicating functionality!

changelog:

• use er.lua instead of own euclidean rhythm module
• more interesting default parameters