Out of core topology optimisation

[mohamed82008]

Currently the problem sizes we can solve are limited to those that can fit in the RAM. This is a limitation that I don't like. In theory, we can use something mmap (like https://docs.julialang.org/en/v1/stdlib/Mmap/#Mmap.mmap) to map an array to a file in the memory. Then we can loop over the array and Julia will load the data in chunks from the file.