Reorganize functions in SynthSeg/dcan/paper/*.py scripts (from code review)

[GregConan]

Reorganize and centralize the Python functions currently in multiple scripts located in the SynthSeg/dcan/paper/ directory. Changes suggested during the code review on 2022-02-09:

• There's no reason to keep all of these functions in separate script files, so all of the subsidiary functions should be moved into the top-level Python script file (the one with a main() function)
• Make the different functions different stages that the user could choose to run via an argparse flag
• Pull the measures variable up to the top level because it is used across multiple scripts/functions
• Make an argparse argument/flag for that measures variable which includes all 5 of them by default, but can be limited to only run some of them