visit_cancer: Extract self-reported cancer history
In philipdarke/ukbbhelpr: Helper functions for UK Biobank data

Extracts self-reported cancer history in a "long" format that is easier to work with than "wide" as provided by UK Biobank (NOTE: watch for type coercion of different data types). Wrapper function for visit_mult_array().

1	visit_cancer(visit_data)

visit_data

Data frame/table with UK Biobank data. Must include fields 20001 and 20006.

Data table with the following columns:

eid: UK Biobank identifier.
date: Reported date of diagnosis.
condition: Cancer type coded as in https://biobank.ctsu.ox.ac.uk/crystal/coding.cgi?id=3.
desc: Description of cancer type (added if coding data can be downloaded).
reported: Visit date at which the cancer was self-reported.