[models,configs/models] Cleanup model implementation to re-use all backbones from torchvision; Simplify checkpointer; Remove own implementation of model_zoo; Implement normalization for torchvision-based backbones; Enables pytorch installation on osx

[data.transforms] Refactor to re-use torchvision as much as possible; Harmonize naming convention with torchvision; Completely test this module

[configs] Documented all configuration files; Added script to list/describe/copy configuration files; Re-structured user guide