Rename HCP1200/ to participants/

[mih]

This is closer to what the directory actually contains: all participants. The fact that they make up the 1200 sample is only valid at this point in time. Brought up by @yarikoptic

[mih]

Still undecided how to go about this. One more perspective. The structure of the S3 bucket is like this

s3://hcp-openaccess/HCP_1200/100206/

so it only differs by an underscore compared to the dataset directory naming. I am considering to implement the following changes:

HCP1200 -> participants (rename) [purpose: release independent path]
HCP_1200 -> participants (symlink) [purpose: 1:1 compatibility with bucket layout]

What do you think @yarikoptic ?