Introduce the concept of nodal and modal discretizations

examples/plot-connectivity.py

examples/simple-dg.py

examples/to_firedrake.py

meshmode/discretization/__init__.py

meshmode/discretization/modal.py

meshmode/discretization/nodal.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

test/test_partition.py

test/test_refinement.py