The CMAKE_GUARD for multiple guarding variables requires a
porper boolean statement.

Possible bug in clang is tracked here:
https://bugs.llvm.org/show_bug.cgi?id=43124

Robert K authored 2 years ago and Andreas Dedner committed 2 years ago

use using and SizeType to size_type to adhere to istl conventions

Notice that this also fixes an incorrect `MultiTypeBlockMatrix::field_type`
in a special setting: If you have a nested matrix
`MultiTypeBlockMatrix< MultiTypeBlockVector<MultiTypeBlockMatrix<...>>>`,
the outer `MultiTypeBlockMatrix::field_type` forwards to the `MultiTypeBlockVector::field_type`
which forwards to the inner `FieldTraits<MultiTypeBlockMatrix>::field_type`.

And fix all spelling errors that codespell currently finds.

... because we also specialize `std::tuple_element` and
it derives from `std::tuple` after all.

Corner case that sometimes happens and produced a segmentation fault.

Unused variable warning indicated this oversight

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

Nested sparse matrices were not allowed to have empty rows/cols. By choosing the minimum offset of other blocks, we get an educated guess for these cases.

Before this commit, the pattern was deallocated with the wrong size that created a huge memory leak.

The current behavior (allocating each row separately ) is rather
surprising and unnecessary as the matrix market file stores the
number of nonzeros anyway. Hence we now pass this number to
BCRSMatrix::SetSize to get the most efficient behavior