Add SumFunctionalConstrainedLinearization class

Just as SumFunctional implements the sum of several independent
functionals, this new class SumFunctionalConstrainedLinearization
implements the sum of linearizations (or, equivalently, the
linearization of the sum).
2 jobs for add-sumfunctionalconstrainedlinearization in 5 minutes and 8 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #110440

00:01:18

passed dune:git gcc-8 C++17 #110441

00:05:08