gfak::mmap_open calls open with O_RDWR (read/write access). This fails with a cryptic error when the file is read-only: ``` odgi: /app/odgi/deps/gfakluge/src/gfakluge.hpp:569: size_t gfak::mmap_open(const string&, char*&, int&): Assertion `false' failed. ```