Improved treatment of time shifts

[rmodrak]

For more information see here

https://github.com/rmodrak/mtuq/blob/master/tests/test_time_shifts.py

In particular

    #
    # MTUQ distinguishes between the following different types of 
    # time-shift corrections
    # 
    # - "static_shift" is an initial user-supplied time shift applied during
    #   data processing
    #
    # - "time_shift" is a subsequent cross-correlation time shift applied during 
    #   misfit evaluation
    #
    # - "total_shift" is the total correction, or in other words the sum of
    #   static and cross-correlation time shifts
    #