possum 0.1.0

First release.

• possum_physiology() and possum_operative() score the twelve physiological and six operative variables; possum() and p_possum() turn those scores into predicted morbidity and mortality.
• possum_risk() scores a whole cohort from a data frame in one call. A row with a missing or out-of-range value is returned as NA and reported in a warning, rather than aborting the whole cohort.
• Raw clinical inputs are bounded above as well as below, so a unit-entry slip (haemoglobin in g/L, urea in mg/dL) is rejected instead of being scored in an inflated band.
• Variants: cr_possum() (colorectal), v_possum() (vascular) and raaa_possum() (ruptured aortic aneurysm), with their own scorers where the cut-points differ.