BUG: Fix bug with reading old reports

[larsoner]

Fixes:

E   KeyError: 'Could not open report HDF5 file:\n/home/larsoner/mne_data/derivatives/mne-bids-pipeline/ERP_CORE/sub-015/ses-ERN/eeg/sub-015_ses-ERN_task-ERN_report.h5, Perhaps you need to delete it? Got error:\n\n    Traceback (most recent call last):\n      File "/home/larsoner/python/mne-bids-pipeline/mne_bids_pipeline/_report.py", line 74, in _open_report\n        report = mne.open_report(fname_report)\n      File "/home/larsoner/python/mne-python/mne/report/report.py", line 729, in open_report\n        report.__setstate__(state)\n        ~~~~~~~~~~~~~~~~~~~^^^^^^^\n      File "/home/larsoner/python/mne-python/mne/report/report.py", line 3176, in __setstate__\n        param_val = state[param_name]\n                    ~~~~~^^^^^^^^^^^^\n    KeyError: \'img_max_res\'\n'

or formatted more nicely when added via TDD:

mne/report/tests/test_report.py:1186: in test_report_backward_compat
    with open_report(fname) as r2:
mne/report/report.py:729: in open_report
    report.__setstate__(state)
mne/report/report.py:3173: in __setstate__
    param_val = state[param_name]
E   KeyError: 'img_max_width'

[drammock]

[nitpick] I often wonder why folks avoid things like

state = _backward_compat_map.update(state)
# or
for param, default in _backward_compat_map.items():
    state[param].setdefault(default)

both of which I think yield equivalent results to the pattern implemented here.
Is it just style? Or is there some other reason I'm not aware of?

[larsoner]

The .update works inplace so that won't do what we want (it would modify _backward_compat_map), but the state.setdefault(param, default) is probably okay, I'll push that one! I just don't use it because I see it less and need to check (like I did this time) if it behaves the way you'd expect with regards to actually setting the key in question, etc.

[drammock]

hah. and I remembered it wrong... but if folks used it more maybe I wouldn't have!