Accelerate the assembly

[hammy4815]

Mainly we can multithread the main for loops of assemble. Possibly there are other changes to be made to improve it on the Julia side. It's currently slower than it should be.

[smartalecH]

Adding multi threading support in this case should be easy. But we should do some rigorous benchmarks to...

1. Ensure the current method really is slower than it should be
2. Ensure we are improving things, not making them worse.

The python implementations vectorize the operations. While this requires much more memory, it does allow for better cache management etc by the compiler.