ENH: Better report coverage

Sped up the BEM plotting a bit, and allowed setting the step. @mainakjas see what you think.
hmm ... try this:

```sh
mne report --path MNE-sample-data/ --info MNE-sample-data/MEG/sample/sample_audvis-ave.fif --subject sample --subjects-dir MNE-sample-data/subjects/ --jobs 6
```

it shouldn't be verbose