Support tabmat matrices as DataArrays

[hammer]

https://github.com/Quantco/tabmat defines some matrix types that could be useful to wrap with DataArray.

[keewis]

there's two things we need to do before that can happen: those matrix types need to implement __array_ufunc__ and __array_function__ (or __array_namespace__, once we support that), and xarray needs to figure out how to deal with ndarray subclasses – right now, they're cast to ndarray.