[la.container.common] added ctor taking a DenseMatrix< T >
[la.container.eigen] added several ctors to EigenDenseMatrix
* one taking a MatrixInterface< M > and
* one taking a DenseMatrix< T >
[common.convergence-study] whitespace
[functions.ESV2007] extended signature of Cutoff

And implemented the current variant as a specialization. This allows us to
add a 2nd variant taking a diffusion factor and a diffusion tensor.
[functions.ESV2007] added first draft of a 2nd variant to playground

This one takes a diffusion factor and a diffusion tensor.