🐛 FIX: Bands tutorial raises two warnings

[giovannipizzi]

Code needs to be updated for this.

See also comments in #263

/home/max/.virtualenvs/aiida/lib/python3.6/site-packages/plumpy/ports.py:194: UserWarning: the validator `validate_protocol` has a signature that only takes a single argument.
    This has been deprecated and the new signature is `validator(value, port)` where the `port` argument will be the
    port instance to which the validator has been assigned.
  warnings.warn(VALIDATOR_SIGNATURE_DEPRECATION_WARNING.format(self.validator.__name__))

and

/usr/lib/python3.6/_collections_abc.py:841: MatplotlibDeprecationWarning: 
The text.latex.preview rcparam was deprecated in Matplotlib 3.3 and will be removed two minor releases later.
  self[key] = other[key]

[mbercx]

@ramirezfranciscof can you check that this is still the case when running the band structure work chain in "running calculations"?