Exercise sheet 10: DGEMM challenge

@apietak @gfederic @jbachmann @lbrodbeck @mkryukov Hi! you can find my attempt at DGEMM challenge in week10/DGEMM_challenge. I have uploaded two versions mm_blocks and mm_blocks_stl. they are the same function but the latter one uses some stl algorithms. Both functions need the functions in the file matrix_operations.cpp in order to work.