add a new property method:
hra_math.model.data_vector.DataVector.is_empty(self)
	which returns true if a signal is None or has no elements

add a new (optional) parameter for poincare plot generation named
-output_prefix (--output_prefix)
it is a label included in a name of an output file,
changes in module hra_math.time_domain.poincare_plot.poincare_plot
output_prefix parameter is managed in the following classes:
hra_core.io_utils.CSVFile
hra_math.model.parameters.file_data_parameters.FileDataParameters
hra_math.time_domain.poincare_plot.poincare_plot_generator.PoincarePlotGenerator
hra_math.utils.io_utils.NumpyCSVFile

add a new function:
hra_math.utils.io_utils.save_arrays_into_file(filename, *arrays)
    which saves collections of arrays as numpy arrays into a file;
    arrays need not to be an equal size, shorter arrays are align
    to maximum sized array by adding 0;
    all data are saved as 15 character long strings

checks in class 
hra_math.time_domain.poincare_plot.poincare_plot.PoincarePlotManager
during generation of a output statistic file or a movies
whether a signal array is None or has no elements, in this case
processing is interrupted