[Constraints] Adapt to new PDELab runtime API for controlling constraints caching
[GridOperator] Export types for vectors, matrix and pattern from GridOperator

The OneStepGridOperator from PDELab wants those typedefs on the
individual GridOperators, so make it happy...
[GridOperator] Store instance of matrix backend and make it accessible

The matrix backend must be available for the new matrix construction method.
[Constraints] Get rid of NoConstraintsCachingPolicy

NoConstraintsCachingPolicy is not required anymore, as the caching
decision now happens dynamically at runtime.
[Jacobian] Replace etadd() with new scatter_jacobian() API

scatter_jacobian() correctly handles the new way constraints caching works.
[testinstationarypoisson] Use new unified qkfem instead of order / dimension specific variants
[testinstationarypoisson] InstationaryPoisson now takes the integration order as a runtime parameter