[script.*] Change prediction to write dataset predictions on a subfolder named after the dataset name in the current run

[script.analyze] Issue warning if keys do not match on for second-annotator evaluations (should not happen, normally)

[script.evaluate] Fix second-annotator comparisons with a key-resemblance check; Add assertions in engine.evaluator to ensure proper comparisons

[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

[script,engine,test] Allows the user to specify the number of steps for threshold/performance analysis

[script.binseg] Add some utility functions to download URLs to temporary files and save command-lines

[script.*] Implement F1-score a priori where possible; Improve plotting of precision-recall curves with better legends, more consistence; Remove redundant plotting code; Update tests