change flag/create a copy themselves

are read-only, so we needd to create copies.

data. Will prpbably need tweaking.

check data shape, and raise if bad.

files. Adjustment to gifti.relatedFiles.

the vertex set changes.

double-barrelled file extensions.

nomenclature.

convinced that this is foolproof, but it seems to work.

by GiftiSurface. Adding comments to confusing model fit code to try
and make refreshing our memories easier.

Used by new gifti function relatedFiles

kept in these classes, instead of in FSLeyes.

of (M*3,). New gifti module for basic gifti surface support.