add all fields but fetch some

[satra]

currently datalad ukb-init -d . <eid> field1 field2 allows adding all fields a project has access to to a participant. however, we may only want to download incrementally. it would be nice if ukb-update could process all by default but specified fields if asked to.

also an option to add fields after init would be nice. easy to do by going and changing .ukbbatch, but a helper may be useful.

[mih]

currently datalad ukb-init -d . <eid> field1 field2 allows adding all fields a project has access to to a participant. however, we may only want to download incrementally. it would be nice if ukb-update could process all by default but specified fields if asked to.

This would require a substantial change in behavior, but is generally doable.

also an option to add fields after init would be nice. easy to do by going and changing .ukbbatch, but a helper may be useful.

FTR: ukb-init --force can replace a list, but not amend one.