nucleotide-count: convert the class to a function

[siebenschlaefer]

Before PR #550 the constructor was required to validate the input, the member function count(char) had to return the count for a specific nucleotide, the member function nucleotide_counts() had to return a std::map with all four counts.

But PR #550 has changed that significantly. After syncing the tests with the canonical data there's no call to count(char) anymore, so each test looks like a two-step function call:

const nucleotide_count::counter dna("G");
const auto actual = dna.nucleotide_counts();

I'd call that unnecessarily complicated and unidiomatic, there's no longer a good reason to require a class. Let's turn the class counter into a function:

const auto actual = nucleotide_count::count("G");

[vaeng]

I really like that change. I remember that felt forced. As if you had to use OOP no matter what.