Physical units of each dimension of the domain.

The physical unit for a dimension is the physical quantity corresponding to a single index increment along each dimension.

A value of None indicates that the unit is unknown/unconstrained. A dimension-less quantity is indicated by a unit of ts.Unit(1, "").

When creating a new TensorStore, the specified units may be stored as part of the metadata.

When opening an existing TensorStore, the specified units serve as a constraint, to ensure the units are as expected. Additionally, for drivers like neuroglancer_precomputed that support multiple scales, the desired scale can be selected by specifying constraints on the units.

>>> schema = ts.Schema()
>>> print(schema.dimension_units)
None
>>> schema.update(rank=3)
>>> schema.dimension_units
(None, None, None)
>>> schema.update(dimension_units=['3nm', None, ''])
>>> schema.dimension_units
(Unit(3, "nm"), None, Unit(1, ""))
>>> schema.update(dimension_units=[None, '4nm', None])
>>> schema.dimension_units
(Unit(3, "nm"), Unit(4, "nm"), Unit(1, ""))