- Support for multi-instance data records (i.e. imaging data from multiple
visits). Data is now organized into `instance-?/` subdirectories, where `?`
is the instance ID (e.g. 2 or 3). In BIDS layout the same IDs are used as
'session' identifiers. Data are now unconditionally grouped by
instance/session, even if no multi-instance data is present yet, in order to
improve forward-compatibility with future UKB updates.
- Availability of `ukbfetch` is now tested at the start of `ukb-update` to
prevent a dirty crash during processing.
- Prevent crash of `ukb-init` with a different set of record IDs.
- Fix log setup to make user-targeted messages visible.