ENH: Minor fixes to add

[larsoner]

Just so I don't forget, from #607

• [x] fix enumeration of example capabilities, https://mne.tools/mne-bids-pipeline/examples/ds003104.html should not have time-by-time decoding: #606
• [x] Add proj joint plots to report using ECG/EOG Epochs: #614
• [x] cache freesurfer/_02_coreg_surfaces.py: #615
• [x] cache _01_make_bem_surfaces.py steps by dealing with recomputation params properly: #615
• [x] cache _03_make_cov.py: #615
• [x] cache _02_make_forward.py by splitting trans step out separately (maybe?): #616, #618
• [x] cache _04_make_inverse.py: #618
• [x] Fix config.* use inside functions, or at least ensure caching is not broken with them: #635
• [ ] Use return_candidates=True in find_empty_room if available
• [ ] cache source/_99_group_average.py
• [ ] cache sensor/_99_group_average.py
• [ ] cache report
• [ ] Add re-run test checks on CircleCI