fix unclosed files (#751, #755, #756, #757)

[a-szulc]

changed all examples of

params = json.load(open(os.path.join(path, "params.json")))

and similar,

to

with open(os.path.join(path, "params.json")) as file:
    params = json.load(file)

[pplonski]

Good catch 👍