[GUI] Flat time series causes error message in logger window

[eneelo]

Describe the bug
Plotting a flat time series in the GUI causes multiple error messages in the logger window, because it fails to produce maxima and cycle distributions.

To Reproduce
Enclosed script generates a time series file (.ts) that with two series that reproduce this behavior and one that does not.
See generate_series.zip.

Expected behavior
If unable generating maxima or cycle distribution fails, the error message should be suppressed and replaced by a warning, e.g. WARNING: could not produce maxima distribution for 'series1'.

Screenshots

Desktop (please complete the following information):
Tested with qats version 5.1.2 on Windows (but will be the same on Linux and with previous qats versions).