Make separate (standalone) modules for each of the model types

[AuguB]

For example, our tree could look something like this:

pcntoolkit
|----__init__.py
|----normative.py
|----normative_utils.py
|----model
|   |----__init__.py
|   |----BLR                   <--- Each model module is standalone
|       |----__init__.py
|       |----blr.py
|       |----blr_utils.py
|   |----GPR
|       |----__init__.py
|       |----gpr.py
|       |----gpr_utils.py
|       |----cov.py
|   |----HBR
|       |----__init__.py
|       |----hbr.py
|       |----hbr_utils.py
|       |----parambuilder.py
|       |----prior.py
|       |----parameterization.py
|   |----...
|----normative_model
|   |----__init__.py
|   |----norm_base.py
|   |----norm_blr.py
|   |----norm_gpr.py
|   |----norm_hbr.py
|   |----...
|----dataio
|   |----__init__.py
|   |----dataloader.py
|   |----dataloader_utils.py
|----preprocessing 
|   |----basis_expansion.py
|   |----scaler.py