built-in configs/plugins.

collapse across visits (although they seem to be lumped into visit 0 anyway).

custon cleaning rules. loadTables returns unknown variables along with
variables that had no cleaning or processing rules, and are not in any
category.

Rf/use schema

Closes #4

See merge request fsl/ukbparse!114

merge them in with the schema base files. Probably broken.

the fly from the (added) UKB schema files

multiple files are accepted, and ukbparse will merge them all. Also removed
--icd10_file option