ENH: Improve Covariance.__repr__

[larsoner]

Improves cov.__repr__. On this branch:

$ python -c 'import mne; cov = mne.read_cov(mne.datasets.sample.data_path() / "MEG"/ "sample" / "sample_audvis-cov.fif"); print(cov); print(cov.as_diag())'
<Covariance | kind : full, shape : (366, 366), range : [-1.3e-11, +5.1e-11], n_samples : 15972>
<Covariance | kind : diagonal, shape : (366,), range : [+6.9e-27, +5.1e-11], n_samples : 15972>

On main:

Too many lines to leave uncollapsed

$ python -c 'import mne; cov = mne.read_cov(mne.datasets.sample.data_path() / "MEG"/ "sample" / "sample_audvis-cov.fif"); print(cov); print(cov.as_diag())'
<Covariance | size : 366 x 366, n_samples : 15972, data : [[ 2.27235589e-23  4.79818505e-24  7.12852747e-25 ...  4.85348042e-18
   2.02846360e-18  8.26727393e-18]
 [ 4.79818505e-24  5.33468523e-24  1.80261790e-25 ...  2.33583009e-19
  -6.93161055e-19  2.35199238e-18]
 [ 7.12852747e-25  1.80261790e-25  5.79073915e-26 ...  1.09498615e-19
   6.16924072e-21  2.93873875e-19]
 ...
 [ 4.85348042e-18  2.33583009e-19  1.09498615e-19 ...  1.40677185e-11
   1.27444183e-11  1.08634620e-11]
 [ 2.02846360e-18 -6.93161055e-19  6.16924072e-21 ...  1.27444183e-11
   1.59818134e-11  8.51070563e-12]
 [ 8.26727393e-18  2.35199238e-18  2.93873875e-19 ...  1.08634620e-11
   8.51070563e-12  1.53708918e-11]]>
<Covariance | diagonal : 366, n_samples : 15972, data : [2.27235589e-23 5.33468523e-24 5.79073915e-26 1.95465404e-23
 9.58056792e-24 3.60094348e-26 1.81981544e-23 1.34520926e-23
 3.06557696e-26 2.96882253e-23 6.52643398e-24 5.74491445e-26
 2.10225446e-23 1.68031534e-23 2.97436798e-26 2.08965492e-23
 1.47060609e-23 2.62781286e-26 1.84361073e-23 1.81840095e-23
 2.19184090e-26 1.88975197e-23 2.68800602e-23 2.56688768e-26
 1.60634828e-23 1.39484233e-23 2.54191786e-26 1.65258130e-23
 1.42576000e-23 2.00152485e-26 1.73383042e-23 1.12707404e-23
 1.34510005e-26 2.02738869e-23 1.53059309e-23 2.90692812e-26
 2.13733254e-23 1.11217011e-23 1.98290428e-26 2.70944458e-23
 1.78858400e-23 1.52557530e-26 2.64442354e-23 1.28272646e-23
 2.27045692e-26 3.46934819e-23 1.70654883e-23 1.85896782e-26
 1.44298685e-23 1.29069385e-23 3.42855070e-26 1.64928823e-23
 1.04175805e-23 4.02816177e-26 9.66863287e-24 9.46040259e-24
 1.90930716e-26 9.76039796e-24 1.21617443e-23 1.67574730e-26
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 1.38443574e-26 9.06976523e-24 1.28314134e-23 7.89148043e-27
 1.41795962e-23 1.00275767e-23 1.79369674e-26 1.44520077e-23
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</details>

[drammock]

I think bandit will complain about assert appearing outside a test file (but maybe not?)

[drammock]

never mind, bandit does complain but it's considered "low" severity so the job still passes. And I don't mind having this kind of assert usage in our code anyway.

[larsoner]

I think we ignore that rule because we do this so often