Implement gap filling functionality
Created by: Dreamlone
Added a file utilities/ts_gapfilling.py. It contains two classes SimpleGapfiller and AdvancedGapfiller, which allow to restore values in one-dimensional time series. some info about the main principles in this classes: for example the "inverse_ridge" function from AdvancedGapfiller works as follows
Added an example to examples/time_series_gapfilling_example.py, where you can try to restore omissions on simulated data. I also added a dataframe to the /data and file with gapfilling case in time_series_gapfilling.py which contains an example of restoring a 1000-element gap in the time series of air temperature from a weather station in Saint Petersburg.