This saves almost 40% of run time.

This unifies the doxygen headers. If the (doxygen) header is not
starting with \file, the following documentation is attributed to a
different entity resulting is multiple copyright blocks and the like in
the doxygen's html files.

md

Functions are only used internally in the translation unit.
No need to expose the interface.

Return immideately.

Using std::vector local variable avoids possible
use after move: access to header.n_cols and n_rows
in the same line as std::move(header).

boost::none -> std::nullopt
boost::make_optional -> std::make_optional
boost::optional<T>::get() -> std::optional<T>::value()

The fmt library happily accepts std::string without
any conversions.