Utilisations

Hi Dariusz,

I have here quite a minor modification. It adds the ability to calculate the utilisations of each state within each component. I'm sure it is not particularly fast. I've also done this in pepa_model.py, I originally had it in solvers/solution.py but I believe it is better here since it doesn't really depend upon the solver. Although this wouldn't necessarily work if we were to use an entirely separate solver technique, such as ODEs or stochastic simulation. In that case this would simply be a method that wasn't really appropriate to use.

Also apologies for lumping an unrelated change in here as well, but it's a very minor modification to the README to remove the 'colorama' from the `pip install` line of the installation instructions since I saw that you removed it as a dependency.

kind regards,
allan