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Let mutligridstep take over ownership of transfer operators if handed over in shared pointer

See merge request agnumpde/dune-solvers!20

The indention before was inconsistent anyway...

This reverts commit a7e17300.

Before you could only set this from a vector of (smart) pointers

There's no need to have non-const transfer operators in the
multigrid step

Feature/resize in matrix vector

See merge request !19

Feature/energynorm matrix provider

See merge request !18

This additionally allows to hand over LinearIterationSteps
that are not equipped with a default BitVectorType.

Internally, we no longer hold the iteration step pointer or
matrix pointer but a matrixProvider that retrieves the matrix
depending on how it was set and can be held using type erasure.