Improve loopy codegen determinism

See merge request !37

Use an ordered set for collecting operators to ensure deterministic
variable naming, as the variable name depends on the collection order.
(As an alternative I tried sorting the output of the collector by
string, but the stringification is very expensive on simple examples
like wave-min.)

Also, improve debuggability of any remaining bugs in loopy kernels
(such as any latent nondeterminism) by giving each assignment kernel a
separate name.

Avoid using specific Py3 sub-versions for CI

See merge request !34

Fix ToLoopyExpressionMapper for subscripted variables

See merge request !32

Caches for speedy bind

See merge request inducer/grudge!30

Placate flake8 3.6

See merge request !28

Add Pytest/JUnit/Gitlab integration

See merge request !27

Preinstall pybind11 for pyopencl

See merge request !25

Use mpi4py.run in spawned MPI subprocessess

See merge request !24

This reverts merge request !23

Add PY_EXTRA_FLAGS="-m mpi4py.run" to help avoid stuck MPI-based CIs

See merge request inducer/grudge!23

Fix one aspect of 1D support

See merge request inducer/grudge!22

Fix env passing throug mpiexec

See merge request inducer/grudge!21

Mpi communication

See merge request inducer/grudge!11