Delete Doxygen blocks duplicated in both .h and .cpp files.
Fix incorrect argument names in \param blocks. Fix filenames
in \file blocks. Fix broken Doxygen syntax (broken numerated
lists containing more than 9 items, invalid HTML tags, broken
links in \ref and malformed math equations). Wrap code blocks
inside \code ... \endcode.

On x86_64, this is always 128 bits, rendering it incompatible with AVX.
GCC reports the maximum enabled vector width (which is what we want) and Intel reports the maximum supported vector width (which is okay too).

This prevents possible deadlocks during reduction at the cost of an
additional communication.