icd9AddLeadingZeroes: Add leading zeroes to incomplete ICD codes
In jackwasey/icd9: Tools for Working with ICD-9 Codes, and Finding Comorbidities

Non-decimal ICD-9 codes with length<5 are often ambiguous. E.g. 100 could be 1.00 10.0 or 100 if coded incorrectly. We must assume 100 is really 100

icd9AddLeadingZeroesShort(icd9Short)

icd9AddLeadingZeroesDecimal(icd9Decimal)

icd9AddLeadingZeroes(icd9, isShort)

`icd9Short`	is a character vector of ICD-9 codes. If fewer than five characters is given in a code, then the digits are greedily assigned to hundreds, then tens, then units, before the decimal parts. E.g. "10" becomes "010", not "0010"
`icd9Decimal`	character vector of ICD-9 codes. If fewer than five characters is given in a code, then the digits are greedily assigned to hundreds, then tens, then units, before the decimal parts. E.g. "10" becomes "010", not "0010"
`icd9`	is a character vector or factor of ICD-9 codes. If fewer than five characters is given in a code, then the digits are greedily assigned to hundreds, then tens, then units, before the decimal parts. E.g. "10" becomes "010", not "0010".
`major`	character vector of 'major' part of ICD-9 codes, i.e. that part which falls before the decimal point, in decimal notation. (In 5 digit notation, the 'major' part is be the first three characters (with leading zeroes), and includes V or E prefix. xyz