Global variables are not shared with executors when run with multiprocessing

[chris-zen]

When running it in MacOS, the multiprocessing module doesn't work as it would be expected in Linux, where the executors inherit the global variables from the parent process. But instead they appear as None, which makes the program to crash. A way to avoid the problem is to configure the OncodriveFML class to avoid parallel by setting the self.avoid_parallel = True in the constructor, and then it runs successfully.

The solution will require to eliminate all the global variables that are needed by the executors, and pass them to every executor explicitly as parameters.

[FerriolCalvet]

See #9 for an example of the errors it does not even work with the example data.