Add line breaks.
Remove indentation as Doxygen treats it as <pre>.

And fix all spelling errors that codespell currently finds.

See https://reuse.software/ for a description.

The previous convergence limit was originally determined experimentally
as 1e-11.  This worked for many blas implementations and architectures.
However, when used with openblas on skylake, apparently the residual norm
would not go below ~1e-10, so convergence was never achieved.  In fact, even
on non-skylake the residual norm would go above 1e-11 again after briefly
dipping below, if iterating further.

We believe that this is due to openblas selecting -- at runtime -- some
skylake specific algorithm leading to a different ordering of operations, in
turn leading to differences in numerical cancellation.  We have however not
verified this conclusively, nor have we identified precisely which blas
algorithm is causing this.

This patch raises the convergence limit to
`sqrt(numeric_limits<field_type>::epsilon())`.  This limit has no theoretical
justification -- it was selected because it usually works as a convergence
limit for other (completely unrelated) algorithms, and because it works for
both Skylake and other architectures (AMD Epyc) in this particular case.

Developed together with Sebastian Westerheide.

Fixes: #48.

The overload of `resize()` with explicit `copyOldValues` argument is easy to
confuse with `std::vector`s `resize()` with default-value for newly created
elements.  In any case, it does nothing here because the BlockVector is empty
before the resize.

- create dedicated "eigenvalue solvers" group
- move implementation details into the "Impl" namespace
- minor cleanups of doxygen documentation

Fixes dune-common#23

The tests often times out when compiled without optimizations

There are some tests, which are hard to port in the paamg directory.
They want to explicitly test _without_ direct solver. Previously,
you simply did not add the flags for the direct solver packages.
add_dune_test relies on all flags, so we need a way to disable components
again. That will be implemented as part of dune_add_executable. For
now, we build the targets ourselves and hand them to dune_add_test.

There are some tests, which are hard to port in the paamg directory.
They want to explicitly test _without_ direct solver. Previously,
you simply did not add the flags for the direct solver packages.
add_dune_test relies on all flags, so we need a way to disable components
again. That will be implemented as part of dune_add_executable. For
now, we build the targets ourselves and hand them to dune_add_test.

added tests using the new eigenvalue algorithm class templates to
compute the spectral (i.e. 2-norm) condition number of the system
matrix associated with a 2D Laplace equation discretized using the
classical 5-point finite difference scheme

initial commit of one class template for performing some iterative
eigenvalue algorithms based on power iteration and of one class
template for performing some eigenvalue algorithms provided by the
ARPACK++ library