add switch to use multiprocessing

• The user can specify with use_multiprocessing=True if he intends to use the power of multiprocessing (recommended for smaller data or on large computing systems) or the serial preprocessing (recommended on small systems or when using debug mode).
• Multiprocessing should be disabled in general in DEBUG mode. Add a 2nd parameter use_multiprocessing_on_debug that is used in debug mode.

To check if running in DEBUG mode use:

import sys
if sys.gettrace() is None:
    debug = False
else:
    debug = True

Change of issue's objectives:

• mutltiprocessing shouldn't be disabled on debug. otherwise parallel parts of the code cannot be debugged. Therefore add a 2nd parameter that is used in debug mode.