Warnings when using `Clusters.plot()` to get a time course

[mmagnuski]

For example

clst.plot(dims=['time'], chan='50%')

produces:

c:\Users\mmagn\Continuum\anaconda3\envs\mnedev\lib\site-packages\numpy\core\fromnumeric.py:43:
VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of
lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you
must specify 'dtype=object' when creating the ndarray.
  result = getattr(asarray(obj), method)(*args, **kwds)