domainer.include() added, used on bar plots

Fix #573: `Domainer`s have no way to say "include this point in the domain", so bar plots failed to include `0`. This branch adds a `domainer.include(n)` that will ensure that `n` is in the output domain.

Though you can add whatever points you want, bar plots now include their `_baselineValue`, which is typically `0`.

Close #573.
See baseline comment for Horizontal BarPlot.