Reorganize code and remove unused _BandedBlockMatrix fn

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.37%. Comparing base (466e2cf) to head (3e48258). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #189 +/- ## ========================================== + Coverage 88.12% 88.37% +0.24% ========================================== Files 11 11 Lines 1204 1204 ========================================== + Hits 1061 1064 +3 + Misses 143 140 -3 ```

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