And fix all spelling errors that codespell currently finds.

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

std::fill() requires a forward iterator, so don't use that algorithm.

The class VariableBlockVector::CreateIterator behaves basically
like an STL output iterator, but it didn't quite implement the
required interface.  This patch adds the missing methods and
extends the unit test.

The main benefit of this (besides interface beauty) is that you can
now use std::fill etc. to set the block sizes of a VariableBlockVector.

Formerly, this would throw an exception if

DUNE_ISTL_WITH_CHECKING

is enabled since you must not call a create iterator on an initialized
VariableBlockVector.

Got rid off unused variable warning

[[Imported from SVN: r1248]]

[[Imported from SVN: r1191]]