Add automatic autosave period calculation

[EvilLord666]

Now we are using Threshold for determine if we saved last spectrum or not, but when we have series we could find time delta (TimeSpan) between spectrum save dates.
We should consider following pecularities:

• spectra measurement could be paused for some time, therefore time delta between them could be more then usual threshold, if delta is big we should consider for this spectrum some Default Theshold (should be added to config)