FIX - reducing all dimensions in cluster viz should at least raise a more informative error.

[mmagnuski]

When all dimensions of a cluster are reduced the error that pops up is not very informative.

For example when one has borsar.Clusters with two dimensions, ['chan', 'time'], then doing:

clst.plot(chan=5, time='75%')

produces just one timepoint for plotting and gives the following error:

  File "C:\Users\mmagn\AppData\Local\Temp\ipykernel_20764\2443041672.py", line 1, in <cell line: 1>
    clst_load['maint'].plot(chan=5, time='75%')

  File "c:\src\borsar\borsar\cluster\obj.py", line 624, in plot
    return plot_cluster_chan(self, cluster_idx, dims=dims, vmin=vmin,

  File "c:\src\borsar\borsar\cluster\viz.py", line 238, in plot_cluster_chan
    topo = Topo(show, clst.info, vmin=vmin, vmax=vmax, show=False,

  File "c:\src\borsar\borsar\viz.py", line 73, in __init__
    self.values = self.values[:, np.newaxis]

IndexError: invalid index to scalar variable.

At minimum, this should raise an error erlier, with some explanation.
Another way out is to plot a bar for example. :)