This can be used to compute the L^1 norm of a function, for instance.

This adds LocalBinaryToUnaryElementIntegrand and
local_binary_to_unary_element_integrand.

This implements LocalElementProductIntegrand, which conforms to the new
itnerface.

This renames `LocalVolumeIntegrandInterface<..., 2>` to
LocalBinaryElementIntegrandInterface and defines a new interface which uses
the bind concept: an integrand object needs to be bound to an element
before use.

This renames `LocalVolumeIntegrandInterface<..., 1>` to
`LocalUnaryElementIntegrandInterface` and defines a new interface which
uses the bind concept: an integrand object needs to be bound to an element
before use.

grid_layer