audit co-morbidity mapping changes from year to year about icd

[jackwasey]

related to #60

[jackwasey]

This could be a vignette task for #82 although complicated by the omission of the annual ICD-9-CM lists from the CRAN distribution of the package. The data file is available on github, in the icd10 branch it is called icd10cm_billable, since it is simply a list of lists of the billable codes, one list for each year for which data is available.

If someone wants to do this, it would be a useful exercise to share in the vignette, and I would then include the annual versions of the ICD-9-CM data in the CRAN package itself.

[jackwasey]

icd 4.0 addresses this by including children of all ICD-10 comorbidity maps for each ICD-10-CM year, and WHO ICD-10. This will cover the vast majority of cases, but we could also consider having localized maps, e.g. for France, which have France-specific ICD-10 codes (only, or alongside ICD-10-CM and WHO years).