function `cyc_len()` should take a vector of integers

[RobinHankin]

Look:

> cyc_len(3:6)
[1] (123)
Warning message:
In seq_len(n) : first element used of 'length.out' argument
> cycle(sapply(3:6,cyc_len))
[1] (123)    (1234)   (12345)  (123456)
> 

It is perfectly clear what I wanted in the first call, namely the result of the second.

[RobinHankin]

But I should fix issue #45 first