Use logging and ctx_factory in tests

See merge request !61

* uses logger instead of print
* uses the provided `ctx_factory` instead of `create_some_context`

Add Elementwise Reductions

See merge request !53

Fix boundary emptiness check

See merge request !60

Standardize all boundary DOFDescs to use DTAG_BOUNDARY, rename DOFDesc.is_boundary -> is_boundary_or_partition_interface

Do not add InterpolationOperator around constants

See merge request !54

Fix einsum in RefMassOperator

See merge request !57