add a new property method:
hra_math.model.data_vector.DataVector.time_signal(self)
        helper method which returns a time array if it is not None or a
signal
        array otherwise; the purpose of the method is return not empty
        time array in the case when a signal array is not expressed in
time units

change in segmenter classes (module
hra_math.model.data_vector_segmenter)
use of data_vector.signal array to property method
data_vector.time_signal  to give 
ability proper evaluation of a range of time window data indexes if  a
signal is not expressed
in time unit but separate array for time data is used