Store current state of work when working on many isotherms at a time

[ltalirz]

@dwsideriusNIST reports that the current digitizer is suboptimal when working on many isotherms at a time (say, 50) because there is a significant risk of losing your progress when e.g. accidentally closing a browser window or a computer crash or ...

While this risk can be mitigated by downloading and saving isotherms more frequently, it would be nice to have the option to save one's state of work more permanently - be it somehow in the user's browser or on the server side