allow use of alternative canonical lists of ICD codes about icd

[jackwasey]

This is more-or-less complete in the development icd10 branch. We certainly can treat different variations differently, but this is necessarily hard coded. E.g. some work has been done for ICD-10 WHO version. It would be pretty easy to add a couple of S3 methods to handle any slight differences between ICD-10 WHO and ICD-10-CM USA. Most generic ICD-10 code would just work. Same for national variations, e.g. suppose we want French ICD-10. We could get our data and set the class to be ("icd10fr", "icd10") (and any other characteristics, e.g. short vs long code).

Annual variations in code sets could be handled by classes (not ideal at all), or keeping the existing class structure, and adding an attribute for the year. This is not going to be a complex OO set-up, just enough to keep things simple and clean.