The RNA base string to get the complement of
A string of the complement of the given base, or undefined if the given base was invalid
getRNABaseComplement('A'); //returns 'U'
getRNABaseComplement('U'); //returns 'A'
getRNABaseComplement('C'); //returns 'G'
getRNABaseComplement('G'); //returns 'C'
Given a valid RNA base string, return its complement