Reunify discretization types

doc/discretization.rst

examples/plot-connectivity.py

examples/simple-dg.py

examples/to_firedrake.py

meshmode/array_context.py

meshmode/discretization/__init__.py

meshmode/discretization/connection/direct.py

meshmode/discretization/connection/modal.py

meshmode/discretization/connection/refinement.py

meshmode/discretization/modal.py

meshmode/discretization/nodal.py

meshmode/dof_array.py

meshmode/interop/firedrake/connection.py

meshmode/interop/nodal_dg.py

meshmode/mesh/visualization.py

test/test_array.py

test/test_chained.py

test/test_firedrake_interop.py

test/test_mesh.py

test/test_meshmode.py