Adding progress bars, on the fly plotting/saving and density matrix Observables

[emile-cochin]

This adds:

• Progress bars: implemented in a new src/ file called utilities.jl as a class on which one can iterate and which returns the time step so that we don't have to modify the run_... code too much. For 1TDVP this adds an ETA and for 2TDVP and DTDVP displays the maximum link size. To disable it them one has to add the parameter progressbar=false in runsim.

• "On the fly": also implemented mainly in the utilities.jl file (and a little in each run_...). Used to display results during simulations or to save temporary data. Simply use the onthefly function, which returns a dictionary to be passed in kwarg onthefly= to the runsim function. This dictionary contains in particular the names of the observables to be saved/plotted, the plot in question, the function used to process the observable and make it possible to graph it, a possible point of comparison on the plot and the number of steps to wait between two saves/graphs. All this is then used by the ontheflysave and ontheflyplot functions in the bodies of the run_... functions.

• Documentation: everything added above is detailled under the "Advanced" subsection of Methods

• RhoReduced which is a new Observable that allows to measure several density matrices reduced to 1 or 2 sites.

[tfmlaX]

Thank you Emile for your contributions and new functionalities! Your work is very much appreciated :)