Fix multiprocessing tests in python2.7
Fix multiprocessing tests in python2.7

Closes #28 for python 2.

Apparently, process pools are not contexts in python 2.7.
On the other hand, in py2 you cannot pickle instance methods, so
you have to implement Pool tasks as independent functions.

gsi-upm/senpy,gsi-upm/senpy,gsi-upm/senpy