working multiprocessing

[LaraFuhrmann]

Aim: Allow multiprocessing for the window building (b2w). This is especially interesting for ultra-high-coverage samples.

In master branch, the windows are build one-by-one through a for-loop.

• In each iteration the varialbe counter variable is updated with the potentially extended window length. Probably this could be done in advance before the iteration over the window, one would need to keep the orginal window length and the extended one. So this lines would to be moved before: https://github.com/LaraFuhrmann/VILOCA/blob/6835ab15f8b0f1a0b4dd32687414555bce0475ca/shorah/b2w.py#L147

[LaraFuhrmann]

I tested the branch on various benchmarking dataset, also comparing envp-mode to non-envp. Everything looks good in the results.