The DNA base string to get the complement of
A string of the complement of the given base, or undefined if the given base was invalid
getDNABaseComplement('A'); //returns 'T'
getDNABaseComplement('T'); //returns 'A'
getDNABaseComplement('C'); //returns 'G'
getDNABaseComplement('G'); //returns 'C'
Given a valid DNA base string, return its complement