Option to save .optimize results to a .txt or .json file

[dpanici]

It could be useful to save the optimization results in a friendly format so that it can be read with existing packages, json is one such option I think. We could add a save_json=False flag to optimize that will save a json file with the name of each objective and their min/max/avg values (both normalized and not), so that one need not scrape the print output in order to get that info.

[dpanici]

Should just be modifying ObjectiveFunction.print_value to return the string (if just writing to a .txt file) and/or also having it return a dict with the data, which should be able to be saved as a json, something like adding the following to the print part of the Optimizer.optimize method

string, data = obj.print_value(obj.x(state), obj.x(state0))

with open(json_save_path,"w+") as f:
    json.dump(data,f)

[dpanici]

add extra info for the initial/final obj values to results output from .optimize as well to avoid the extra print_value call