--child_values support for non-numeric data

changes the default behaviour of inferring column types when working with
non-UKB data. Instead, ensure that navalues/rawlevels/etc are coerced to
appropriate type when applied. Also only calculate raw/new level correlation
on numeric columns

otherwuse subsequent assignments (really only in unit tests) become very
awkward. New variableCategories function; no use in code, but useful when
exploring/working with data files

merged - next commit will convert to appropriate type

CI: Thought I might need pandoc too

See merge request fsl/funpack!79

Mnt/pages

See merge request fsl/funpack!78

kernel is installed

Rf/unknowns

See merge request fsl/funpack!77