BUG: subject_info dict validation

[larsoner]

The thing is, I can do something like this:

raw = mne.io.read_raw('valid_data.fif');
raw.info['subject_data']['weight'] = np.array([np.random.rand((200, 20))])

...

Adapted from code by @thht in mne-tools/mne-bids#1310 (comment)

The code above will fail with raw.save I think. We should validate the info["subject_info]" dict like we do for Info. There are maybe some other info fields we should protect at some point but maybe YAGNI and subject_info seems like a clearly good target since we know people change it.

We should also document that it can be changed if we don't already. I think in the docstring of Info we do list what is considered user-modifiable and what is not, but I need to look.