The `MatrixIndexSet` class provides access to an
incomplete subset of its internal data. While
the number of rows is exported, the number of cols
and the column indices per row have been missing
so far. This was an oversight, since one can hardly
use the class without this, unless one relies on
the dedicated `exportIdx()` method for `BCRSMatrix`.

With the missing data exported, one can now easily
use `MatrixIndexSet` to build patterns for
other sparse matrix classes.

Before this patch `BCRSMatrix::addindex()` was used on individual
column indices. This is slow because each insertion does a binary
search although the inserted indices are already sorted. Bulk-inserting
whole rows with `setIndices()` improves on this significantly but still
does a non-necessary sort. The latter is avoided by the new
`BCRSMatrix::setIndicesNoSort()` method.

Using `std::uint_least32_t` instead of `std::size_t` halves the required memory
and thus also improves performance.
Notice that `std::uint32_t` is optional and `std::uint_fast32_t` is intended to be
fast in terms of computations and may have 64 bits. Here we're interested in reducing
memory and thus bandwith. Hence `std::uint_least32_t`, which is the smallest sufficient
type is most appropriate.

This replaces the `std::set` used to store the column indices for each
rows by a `std::vector` based implementation. The `std::vector` is
kept sorted when inserting such that we can avoid duplicates. This
can improve assembly time of matrices significantly (if `MatrixIndexSet` is used).

In the worst case (insertion in reverse) this may lead to O(n^2) complexity
when inserting n entries in a row compared to O(n log(n)) for `std::set`.
However, the sorted `std::vector` implementation still wins for relatively
large n. To avoid the worst case complexity when using very dense rows,
the implementation is switched to `std::set` if the size exceeds the threshold
value `maxVectorSize`. The default `maxVectorSize=2048` was selected based
on benchmark results.

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

[[Imported from SVN: r1545]]

Thanks to Uli Sack for the Patch.

[[Imported from SVN: r1544]]

[[Imported from SVN: r1120]]

* update configure and Makefiles

[[Imported from SVN: r1100]]

[[Imported from SVN: r773]]

[[Imported from SVN: r674]]

[[Imported from SVN: r669]]

has to be adapted.

[[Imported from SVN: r656]]

[[Imported from SVN: r528]]

[[Imported from SVN: r314]]

[[Imported from SVN: r248]]

contain any entries at all.  This is needed because currently BCRSMatrix
doesn't allow rows that are completely empty.

[[Imported from SVN: r147]]

[[Imported from SVN: r141]]

a set of (i,j) integer pairs.  Pairs can be added to the matrix
in any order.  A MatrixIndexSet can be used to initialize a
BCRSMatrix via the exportIdx method.

MatrixIndexSets are meant to help with the sometimes unwieldy
BCRSMatrix class.  If that class' handling gets easier, (or
if somebody explain to me how BCRSMatrix are really supposed
to be used), MatrixIndexSets may very well disappear again.

[[Imported from SVN: r134]]