ENH: Better report coverage

Sped up the BEM plotting a bit, and allowed setting the step. @mainakjas see what you think.
That's really disappointing -- sounds like a good reason not to use `pylab` mode for development. It also probably decreases efficiency to use list comprehension in general (although it's going to be trivial in this particular instance). Do you know of mne-python devs actually using this mode? If so I'll change it, but if not it should be cleaner to leave as-is.