moved Elixhauser comorbidity calculation from icd9.R to comorbid.R
numerous corrections to the Elixhauser co-morbidities. Originally based off 'comorbidities' package, now corrected based on original Elixhauser paper, including V codes and .0 and .00 changes which can result in different codes when omitted.
minor updates to datadocs, vignette and tests