[assembler.system] minor update
[spaces.constraints] introduced CRTP interface

Unfortunately there is no way around this: ConstraintsInterface now is a
CRTP interface. Otherwise template matching of the local_constrains()
method of a space would break, since the wrapper of the constraints used
in the system assembler would erase the derived type. New constraints can
easily be added with a minimum amount of boiler-plating, though, using
Constraints::internal::Default as a base, see Constraints::Dirichlet for
example.
[spaces.interfaces] updated constraints
[spaces.cg.bsae] updated constraints
[assemlber.wrapper] clean up and whitespace
[assembler.wrapper] updated LocalMatrixConstraintsWrapper
[assembler.system] updated add() for local matrix constraints