Handle case if benchmark results contain only one simulation

[jhnnsnk]

Raises currently an IndexError:

Traceback (most recent call last):
  File "../analysis/analysis.py", line 50, in <module>
    plot(
  File "/p/project/cjinb33/jinb3320/repositories/beNNch/analysis/plot_helper.py", line 59, in plot
    B.plot_fractions(axis=ax1,
  File "/p/project/cjinb33/jinb3320/repositories/beNNch/plot/benchplot/bench_plot.py", line 247, in plot_fractions
    axis.fill_between(np.squeeze(self.df[self.x_axis]),
  File "/p/software/jurecadc/stages/2020/software/SciPy-Stack/2021-gcccoremkl-10.3.0-2021.2.0-Python-3.8.5/lib/python3.8/site-packages/matplotlib-3.3.1-py3.8-linux-x86_64.egg/matplotlib/__init__.py", line 1438, in inner
    return func(ax, *map(sanitize_sequence, args), **kwargs)
  File "/p/software/jurecadc/stages/2020/software/SciPy-Stack/2021-gcccoremkl-10.3.0-2021.2.0-Python-3.8.5/lib/python3.8/site-packages/matplotlib-3.3.1-py3.8-linux-x86_64.egg/matplotlib/axes/_axes.py", line 5301, in fill_between
    return self._fill_between_x_or_y(
  File "/p/software/jurecadc/stages/2020/software/SciPy-Stack/2021-gcccoremkl-10.3.0-2021.2.0-Python-3.8.5/lib/python3.8/site-packages/matplotlib-3.3.1-py3.8-linux-x86_64.egg/matplotlib/axes/_axes.py", line 5229, in _fill_between_x_or_y
    for idx0, idx1 in cbook.contiguous_regions(where):
  File "/p/software/jurecadc/stages/2020/software/SciPy-Stack/2021-gcccoremkl-10.3.0-2021.2.0-Python-3.8.5/lib/python3.8/site-packages/matplotlib-3.3.1-py3.8-linux-x86_64.egg/matplotlib/cbook/__init__.py", line 1253, in contiguous_regions
    idx, = np.nonzero(mask[:-1] != mask[1:])
IndexError: too many indices for array: array is 0-dimensional, but 1 were indexed

Decide if just one data point shall be plotted or if the plotting should fail with a message that the function is not applicable for the provided type of data.