Simplify _indices_and_coefficients to return 0-indexed cols

[odow]

We were previously doing lots of unnecessary - 1 and Cint.().

[codecov[bot]]

Codecov Report

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

Project coverage is 63.60%. Comparing base (00df013) to head (e2b3fc3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #245 +/- ## ========================================== - Coverage 63.73% 63.60% -0.14% ========================================== Files 6 6 Lines 3513 3511 -2 ========================================== - Hits 2239 2233 -6 - Misses 1274 1278 +4 ```

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