add plot function

[brittonsmith]

This adds a plot function as a wrapper around SlicePlot to act as a workaround for the slowness is plotting large datasets. The syntax is:

ds = yt.load(...)
p = ds.plot(('bands', '1'), height=(10, 'km'), width=(5, 'km'), center=ds.domain_center)

or

p = ds.plot(('bands', '1'), data_source=...)

Internally, this creates a dataset with altered domain left and right edges confined to the region being plotted.

I also fixed up a couple of minor things:

• removed some old demo notebooks
• use correct attribute for periodicity
• add custom parameter print to remove irrelevant cosmology parameters