DOC: update apiref stub files

See merge request !106

no idea if it is correct, and no way to test)

ENH: Allow variable categories to be used in processing rules

See merge request !105

e.g. "all,1,2,cat5 someproc()" will apply someproc to vars 1, 2, and all vars
from category 5

Download latest versions of UKB schema files

See merge request !104

NEVER_DOWNLOAD. Functions to [down]load field.txt/encoding.txt

first, just call loadHierarchyFile instead.

call loadCodingFile. Behind the scenes, the coding file is downloaded from
the UKB showcase, and the files in funpack/schema/coding/ used as a backup
if the download fails.

BF: Fix evaluation order calculation in child value replacement

See merge request !103