RFC: better API for plot_topomap bands parameter?

[drammock]

plot_psds_topomap, plot_epochs_psd_topomap, and epochs.plot_psd_topomap have a bands parameter that takes a list of tuples as input. The format is:

Length-2 tuples specify a single frequency and a subplot title (e.g.,
``(6.5, 'presentation rate')``); length-3 tuples specify lower and
upper band edges and a subplot title.

This seems inelegant: input is one or more strings, each associated with 1 or 2 numbers. It is begging to be a dict. Proposed new API:

If a :class:`dict`, keys will be used as subplot titles and dict values should be
either single frequency values or length-two :class:`tuples<tuple>` indicating
lower and upper band edges.

We could easily, silently support 'lists' or np.arrays as the dict values too; and also treat length-1 dict values as equivalent to passing a single value:

if not hasattr(value, '__len__') or len(value) == 1:
    # assume single freq value
elif len(value) == 2:
    # lower/upper band edges
else:
    raise ValueError()