Define consitent module structure

Proposed new structure:

cmrism
+-- auxilliary
|   +-- __ini__.py
|   +-- ...
+-- datasets
|   +-- __init__.py
|   +-- multirep.py
|   +-- bloch.py
|   +-- flow.py
+-- analytic
|   +-- __init__.py
|   +-- contrast
|   |    +-- __init__.py
|   |    +-- diffusion.py
|   |    +-- ...
|   +-- encoding
|   |    +-- __init__.py
|   |    +-- base.py
|   |    +-- cartesian.py
+-- bloch
|   +-- __init__.py
|   +-- base.py
|   +-- gradients.py  
|   +-- readout.py  
|   +-- rf.py
|   +-- readout.py
|   +-- ...
+-- reconstruction
|   +-- __init__.py
|   +-- base.py
|   +-- cartesian.py
+-- simulation.py

Furthermore to shorten import paths, __init__ of datasets, contrast and encoding should include the direct import of the single classed defined in submodules.