Create element group base class to indicate interpolatory-or-not

See merge request inducer/meshmode!21

Same-mesh connection: allow 'interpolating' for non-interpolatory groups if nodes are exactly identical

Fix an unbound variable in ChainedDiscretizationConnection.

See merge request inducer/meshmode!20

Also test ChainedDiscretizationConnection, because it doesn't
look like it is being tested.

Rename FRESTR symbols to something more self-explanatory (thanks @eshoag2)

See merge request inducer/meshmode!19

Change ALL_FACES to FRESTR_ALL_FACES and INTERIOR_FACES to

See merge request inducer/meshmode!18

FRESTR_INTERIOR_FACES in a few documentation spots.

(Noticed by Li Lu.)

Documentation fixes / Discretization doc improvements

See merge request !17

pointing this out).

Add an 'apple' test geometry.

See merge request !16