Change data shape of 1D Dats to (dset.total_size,)

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.
Remove mass2d_triangle and rename mass2d_unstructured regression test

The old mass2d_triangle regression test is removed since its "known
good" reference data is out of date and the mode of testing against
such a reference is not that valuable. So rather than updating the
reference, the test is removed and the old mass2d_unstructured test
is renamed to better reflect the demo it is testing.