Simplify process of using manual ticks

[swharden]

Let's not break the existing API, it's fine, but let's add a shortcut so it's easier to do this:

ScottPlot.TickGenerators.NumericManual gen = new();
plt.Axes.Bottom.TickGenerator = gen;
for (int i = 0; i < 10; i++)
    gen.AddMajor(i, "test");

Maybe we can wrap that functionality in a function like:

double[] xs = [1, 2, 3];
string[] labels = ["one", "two", "three"];
plt.Axes.Bottom.SetTicks(xs, labels);