Make 1D Dats use 1D arrays to store data

Previously, a 1D `Dat` had a shape of `(dataset.total_size, 1)`, which implies it's a column vector. However that is not necessary and makes it harder to use in some cases. The NumPy default to store 1D arrays is a shape of `(dataset.total_size,)`, so use that.

For the regression tests comparing against a "known good" reference, this had to be updated. I think rather than doing that, we should change that regression test to use some other pass/fail metric.