Change hardcoded MSI paths in `SynthSeg/dcan/paper/*.py` scripts (from code review)

[GregConan]

Hardcoded MSI paths suggested to change in the Python scripts located in the SynthSeg/dcan/paper/ directory, all suggested during the code review on 2022-02-09:

In General

• Replace hardcoded MSI paths with input arguments that the user can change

Change hardcoded MSI paths in specific Python scripts

(Scripts in the SynthSeg/dcan/paper/ directory)

generate_metrics.py

• Currently, results_dir is hardcoded
  • To make this script reproducible on different systems, it should just be a hardcoded default option in an argparse parser

create_plots.py

• Change the hardcoded relative path to {measure}.png on line 18 to an input arguments (possibly with a hardcoded default)