Not backward compatible - re-installation is required

* Fixes for PyTorch==1.0.0
* Refactoring folder structure
* Update installation section in docs

Sometimes the gmin/gmax in group color-normalization ends up with a zero
dimensional tensor, which needs to be accessed using .item()

- Raise IOError instead of crude exit() when file is not found in the file-system
- Test that the correct exception is raised when opening a non-existent
checkpoint file