Extension of Kmer class to return numeric indices for each nucleotide

[ghuls]

Extension of Kmer class to return numeric indices for each nucleotide:

@extend
class Kmer:
    def nucleotide_indices(self) -> List[int]:
        """
        Get nucleotide indices for each nucleotide position in the Kmer.

        The nucleotide index for a nucleotide corresponds with the position in "ACGT".
        """
        nuc_idx = List[int](k)
        x = self.x
        i = k - 1
        while i >= 0:
            nuc_idx.append(int(x & UInt[2*k](3)))
            x >>= UInt[2*k](2)
            i -= 1
        nuc_idx.reverse()
        return nuc_idx