clip returns DataArray instead of original type

As of xarray 0.18, the `clip` method uses the `apply_ufunc` function (https://github.com/pydata/xarray/compare/v0.17.0...v0.18.0#diff-5e1c0e7a002249e30f1e688e4e87af2d67600ce8a240f8b27dccdb28f365c971R381) which in turn uses `apply_dataarray_vfunc` and ends up returning a DataArray https://github.com/pydata/xarray/blob/v0.18.0/xarray/core/computation.py#L280.

Is there a way to preserve the assembly's original data type? E.g. if we call `.clip()`, we get `BehavioralAssembly` as output again.

Here's a unit test that fails in 0.18 but works in 0.17:
```
def test_clip(self):
assembly = BehavioralAssembly([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]],
coords={'a': ['a', 'b', 'c', 'd'],
'aa': ('a', ['a', 'a', 'b', 'b']),
'b': ['x', 'y', 'z']},
dims=['a', 'b'])
clipped = assembly.clip(-5, +5)
assert type(clipped) == type(assembly)
```