Update init files for optimized import functionality

[rspwarnaarUT]

Previously this reference:

>>> import resurfemg
>>> resurfemg.preprocessing

Would yield:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'resurfemg' has no attribute 'preprocessing'

By defining subpackages and modules in init.py files, we get a successful import:

>>> import resurfemg
>>> resurfemg.preprocessing
<module 'resurfemg.preprocessing' from 'PATH_TO_LIBRARY\\resurfemg\\preprocessing\\__init__.py'>