Add comments to `SynthSeg/dcan/image_generation.py` describing its purpose (from code review 2022-06-22)

[GregConan]

• Describe the specific purpose that this was developed for, an example use case, and the other ways it can be used
  • Include function-level and module-level description, because even if users shouldn't usually need to look at the code, they may in an error
• Mention
  • that this script generates the distorted/blurred/etc. synthetic T1s and T2s
  • that what the script generates depends on the input parameters and priors
  • what GMM stands for (Guassian Markov Models?)
    • As Steve says, we shouldn't use an acronym unless we know what it means!
• Add comment to line 71 & 73 specifying that the 0000 and 0001 mean T1 and T2