Modify BFL residual

[A-CGray]

Purpose

In the balanced field length group, the decision speed $V_1$ is supposed to be solved for such that the abort and continue distance are equal. A fallback is required if these distances cannot be made equal while satisfying $V_1 \leq V_r$. To handle this, the docstring of the BFLImplicitSolve says the following:

This residual is equal to zero if:
	- The rejected takeoff and engine-out takeoff distances are equal, or:
	- V1 is equal to VR and the engine out takeoff distance is longer than the RTO distance

This PR simply cleans up the implementation of this logic in the code to make it more comprehensible

Checklist

Put an x in the boxes that apply.

• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.05%. Comparing base (0c2f844) to head (94cc1ed).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   82.05%   82.05%           
=======================================
  Files         103      103           
  Lines       10729    10730    +1     
=======================================
+ Hits         8804     8805    +1     
  Misses       1925     1925           

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