This function takes a pedigree and converts it into a standard integer form used by many functions in the 'nadiv' package.
A three column pedigree object, where the columns correspond to: ID, Dam, & Sire
Missing parents (e.g., base population) should be denoted by either 'NA' or '0'.
Individuals must appear in the ID column in rows preceeding where they appear in either the Dam or Sire column.
Based on code from the
The pedigree, where individuals are now numbered from 1 to n and unknown parents are assigned a value of '-998'.