Array context

WIP because:

• Tests need to be adapted (thanks @fikl2 for your help)
  Test status (CI is set up to track the status of ported tests. Add ported tests to setup.cfg.):

  • test_cost_model.py
  • test_global_qbx.py
  • test_layer_pot.py
  • test_layer_pot_eigenvalues.py
  • test_layer_pot_identity.py
  • test_linalg_proxy.py [@fikl2]
  • test_matrix.py [@fikl2]
  • test_maxwell.py
  • test_muller.py
  • test_scalar_int_eq.py [@mattwala]
  • test_stokes.py [@fikl2]
  • test_symbolic.py
  • test_target_specific_qbx.py
  • test_tools.py
• Revert changes to setup.cfg once all tests are ported
• Revert changes to .gitlab-ci.yml
  • Revert commented out CI jobs
  • Re-disable tests marked slowtest in Python 3 POCL job
• Examples need to be adapted
• FMM calls need to be adapted
• Address all FIXMES added during test porting
• meshmode!80 (merged) should go in at the same time
• Check requirements.txt and .test-conda*.yml for things to re-dangle

Related: !201 (merged)