Split up connectivity.mat file to follow BEP017

[tsalo]

Summary

BEP017: Relationship matrices defines how correlation matrices should be defined. We can still compress the relmat files if we adopt BEP017, whether with tsv.gz, zarr, h5, or parquet.

[mattcieslak]

one possible gotcha here is that a very large number of these matrices get created. Each atlas gets a matrix for each weighting type (eg sift, average scalar, etc) and connectivity calculation method (pass, end, etc).

[tsalo]

It's a hard one. In XCP-D there are a bunch of correlation matrices that we just put in separate files. We end up with a lot of files, but it's still manageable.

[tsalo]

Per #164 (comment), we'll make the split at some point.