InverseGaussian: Improve quantile behaviour.

[jzmaddock]

Provide a better initial guess for the root, and bracket the root to avoid shooting off to infinity. Fixes: #1392 Refs: scipy/scipy#25096

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.35%. Comparing base (ec801eb) to head (54fd419).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1395      +/-   ##
===========================================
+ Coverage    94.36%   95.35%   +0.99%     
===========================================
  Files          704      825     +121     
  Lines        49012    68223   +19211     
===========================================
+ Hits         46249    65054   +18805     
- Misses        2763     3169     +406     

Files with missing lines	Coverage Δ
...lude/boost/math/distributions/inverse_gaussian.hpp	85.33% <100.00%> (ø)
test/test_inverse_gaussian.cpp	100.00% <100.00%> (ø)

... and 165 files with indirect coverage changes

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