[!349] add `exportMatrix()` to `HierachicalLinearOperator`
Merge branch 'feature/hierachical' into 'master'
ref:dune-fem/dune-fem> also needs to be backported to 2.7
See merge request dune-fem/dune-fem!349
(cherry picked from commit 1bc6d26f)
058c65bd add exportMatrix()
to HierachicalLinearOperator
and use it in test