This reverts merge request !564

Remove allocator template parameter from the classes
`base_array_unmanaged`, `compressed_base_array_unmanaged`,
`block_vector_unmanaged`, `compressed_block_vector_unmanaged`, and
`CompressedBlockVectorWindow`.
All of these classes do not manage memory on their own.
The provided allocator was only used to deduce `size_type`.
The latter is now provided as template parameter explicitely.

Notice that this (seemigly breaking) change is safe, because
all these classes are clearly marked as implementation details
that should not be used outside of dune-istl.

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

Using `Impl::asMatrix()` we can replace many special case
implementations based on `Hybrid::ifElse(IsNumber...` which
simplifies code and error messages and avoids some code duplication.

Indeed, isNaN could be something complicated, although normally it just
should be double.

Oliver Sander authored 8 years ago and Ansgar Burchardt committed 8 years ago

There are various classes in dune-istl that are effectively implementation
classes internal to dune-istl. A few of them are base_array_unmanaged,
base_array_window, compressed_base_array_unmanaged,block_vector_unmanaged,
BlockVectorWindow, compressed_block_vector_unmanaged, CompressedBlockVectorWindow

See #22 for a brief
discussion.

This patch moves the classes into the namespace 'Imp'.  That way it is
clear that they are internal.

All code outside of dune-istl using those classes will break, but does
such code really exist?

Due to a confusion between the number of lines N and the number of
columns M, Matrix::transpose() gave wrong results for matrices with
N != M.

I missed a few occurrences of 'nblocks' when renaming that to 'rows_',
because it was hidden behind the DUNE_ISTL_WITH_CHECKING macro.

This array was a left-over from the VariableBlockVector class.  In short,
it stores the length of each matrix row.  Of course that is redundant,
because all rows in a dense matrix have the same length.

This patch makes the class allocate less memory, and use the known row
size to compute the offsets into the data array.  In principle, the code
becomes faster by this, but I am not convinced that the effect will be
measurable.

Still, even without measurable speedup: this patch is an improvement,
because it makes the code simpler.

All blocks have the same size, therefore 'block' is not actually needed.
This is the first step towards getting rid of it.

The recent changes to VariableBlockVector broke the Matrix class -- shame on me
for testing thoroughly enough before pushing.  However, using VariableBlockVector
internally in Matrix was a hack anyway.  So, as a first step towards a cleaner
solution, this patch copies the (old) VariableBlockVector class into the file,
renames VariableBlockVector to DenseMatrixBase, and does some obvious cleanup.
This is still not particularly pretty, but better than before.

The information has been provided by the MatrixDimensions struct since roughly the end of 2010.
Correspondingly, the methods rowdim and coldim have been removed from the istl matrix implementations.
Apparently, we forgot to remove them from the Matrix class.

use FieldTraits to determin the return type of the norm methods

[[Imported from SVN: r1644]]

[[Imported from SVN: r1570]]

Unfortunately DUNE_DEPRECATED does not work before definitions of
template member functions inside the class.

[[Imported from SVN: r1551]]

[[Imported from SVN: r1510]]

[[Imported from SVN: r1509]]

Bugfix: class Matrix does not have a method clear().

[[Imported from SVN: r1507]]

[[Imported from SVN: r1494]]

rbegin -> beforeEnd
rend   -> beforeBegin

[[Imported from SVN: r1414]]

*sigh*

[[Imported from SVN: r1410]]

[[Imported from SVN: r1404]]