File and class organization

I'd rather start with cleanup early. Here are my propositions on how to organize this module:

• Move the following files to subdirectory types
• blockmatrixview.hh
• componentwisematrixmap.hh
• singlenonzerocolumnmatrix.hh
• singlenonzerorowmatrix.hh
• Move the following files to a subdirectoy algorithm (?)
• addtodiagonal.hh
• algorithm.hh (rename to sparseMatrixSomething.hh?)
• axpy.hh (rename to addProduct.hh, maybe also have substractProduct.hh)
• axy.hh
• crossproduct.hh
• genericvectortools.hh (split up functions?)
• getscalar.hh
• transformmatrix.hh
• transpose.hh
• Move the following files to a subdirectory solvers (?)
• ldlt.hh
• triangularsolve.hh
• Optionally move the following files to a subdirectory tools (???)
• algorithm.hh/sparseMatrixSomething.hh (instead of moving it to algorithm)
• concepts.hh (would otherwise remain in dune/solvers-root)
• promote.hh (would otherwise remain in dune/solvers-root)

I propose this also in view of to-be-migrated files from dune-solvers (many of dune/solvers/common), among them being

• defaultbitvector.hh
• resize.hh (provides resizeInitialize)
• genericvectortools.hh
• wrapownshare.hh
• tuplevector.hh
• staticmatrixtools.hh
• copyorreference.hh
• arithmetic.hh

Also, I am not sure about how to go with a transition phase. Introducing forwarding includes like currently there for matrixtraits.hh and scalartraits.hh is probably the most convenient, but I feel like it might spoil the point of cleaning up promptly.

For neither folder I'd introduce dedicated namespaces as of now.