Initializing random_density

[zerothi]

The random_density could be improved slightly

Here are some general thoughts that may be helpful for you when reconsidering what to do:

• It is a little annoying that the random_density prints to std-out. I would simply remove it, generally remove as many print-statements and provide a logger functionality if required (I plan on doing something with the mixer, but I haven't looked at it yet).
• It would be really nice if one could (optionally) add a random_density instead of overwriting it, imagine a converged system, just to give it a bump one could add a random density (with some weight), for instance HH.dm += np.random.rand((2, no)) * 0.1 - 0.05. The thing is that sometimes one is not fully sure the SCF convergence finds the global minimum, for some of your systems I found that switching between Pulay and Linear mixing could result in two different minima. I didn't check their configurations.