omit pointless extra Newton iteration for λ

[stevengj]

Since Newton iterations should be quadratically converging by the time we approach machine precision, the final iteration here is pointless (the next correction should be of order $\varepsilon^2$).

[codecov[bot]]

Codecov Report

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

Project coverage is 98.55%. Comparing base (2e6fc15) to head (5863a04).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ========================================== - Coverage 98.56% 98.55% -0.01% ========================================== Files 7 7 Lines 696 694 -2 ========================================== - Hits 686 684 -2 Misses 10 10 ```

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