GenotypeLikelihoods.calcNumLikelihoods() should not be recursive

[droazen]

GenotypeLikelihoods.calcNumLikelihoods(numAlleles, ploidy), which is the method used to calculate the length of the PL array given the number of alleles and the ploidy, is currently written using an inefficient recursive implementation:

    private static final int calcNumLikelihoods(int numAlleles, int ploidy) {
        if (numAlleles == 1) {
            return 1;
        } else if (ploidy == 1) {
            return numAlleles;
        } else {
            int acc = 0;

            for(int k = 0; k <= ploidy; ++k) {
                acc += calcNumLikelihoods(numAlleles - 1, ploidy - k);
            }

            return acc;
        }
    }

However, this method has an easy closed-form combinatorics solution using the formula for selection with replacement:

So (for example), for numAlleles = 5 and ploidy = 2, it is just (5 + 2 - 1) choose 2 = 6 choose 2 = 15

(Thanks to @fleharty and @kachulis for their help in deriving this!)

Since this method is called every time we create a PL array (which is very frequently!), it seems worth it to get rid of the recursion in favor of a one-line call out to a standard combinatorics function.

[droazen]

For @KevinCLydon as part of his work on https://github.com/broadinstitute/gatk/issues/6322