lists in data frames, and merging dataframes with different index dtypes

improve/fix associative test

function. Implemented for pairwise version, need to do matrix version.
Change to processing core function signatures

Bf/empty columns

See merge request !81

Update cleaning options to support non-numeric

Closes #23

See merge request !80

--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