Adding TimeSeries class

Adding TimeSeries class that holds time series data. Time series by default are held in pandas::DataFrame with index corresponding to the channel. In the specific case of seismic time series, there's a child class SeismicTimeSeries and the data is held in ObsPy's stream object, which allows to use all ObsPy methods on it. It can be still converted to common pandas::DataFrame format, and it is converted internally for instance to calculate (L)PSD.

Bug fixed with pandas::DataFrame format time series: in this case, index or time counts are relative time, i.e. starts from zero. When files are stitched together, this index started from zero once again, now it starts from the last value of the previous file plus one period.