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 inducer/grudge!54

Fix einsum in RefMassOperator

See merge request inducer/grudge!57

* skip when the dd_in == dd_out
* skip for constants

Allow POCL CI failures for now

See merge request inducer/grudge!59

Add atan2 FunctionSymbol

See merge request inducer/grudge!55

execution: fix map_if for scalars

See merge request inducer/grudge!52

1D Fixes

See merge request inducer/grudge!48

Remove cyclic dependencies in symbolic module

See merge request inducer/grudge!49

The problem is that in 1D faces are 0D and so carry exactly no
useful information about which side of the element they may be
on.

To hack around that, this commit introduces a hacky operator
that hacks it. Less ambiguously, we just use the connection to
figure out which face we're on on set the normal to `+1` or `-1`
based on that