docs - complete description of code snippets for ROI-based computations

[steelec]

i created an initial stump of a wiki file to describe this process, would be great if others could contribute to complete it!

[zklsmr]

Completed description on extracting summary statistics and mapping values to original space. Needs verification by someone and then we can close this issue.

[steelec]

great start, lets ping @ajhelum in here to add some descriptive text around the code snippets to explain what the process is and what you get at each step

including link here, since I see no way to include references to the wiki,
https://github.com/neuralabc/documentation/wiki/Generating-parcels-and-computing-summaries-for-subsequent-analysis

[ajhelum]

I will look into it, and add the descriptions.