Co-morbidity handling much improved.
 * Bump version in anticipation of a release.
 * Specific functions for applying mappings to lists of icd9 codes, one for each co-morbidity, each performing mapping-specific details like not double counting mild and severe liver disease. Will someday be relatively easier to convert this to inherited classes but at present this is not worth the clumsiness of doing it in R S4/5 objects.
 * pre-set naming scheme: considerable overlap for the Elixhauser-based mappings. Have provided full and short names for each co-morbiditity. Several permutations because of HTN handling.
 * resaved data files and updated tests using the standardized the naming scheme
 * tweaked documentation in R/convert.R