Fix skew normal by exp-sinh quadrature

[NAThompson]

Closes #1190

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.41%. Comparing base (c1acadb) to head (c5adf11).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/math/distributions/skew_normal.hpp	96.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1449   +/-   ##
========================================
  Coverage    95.41%   95.41%           
========================================
  Files          831      831           
  Lines        69550    69575   +25     
========================================
+ Hits         66359    66384   +25     
  Misses        3191     3191           

Files with missing lines	Coverage Δ
test/test_skew_normal.cpp	100.00% <100.00%> (ø)
include/boost/math/distributions/skew_normal.hpp	85.77% <96.00%> (+1.19%)	⬆️

... and 1 file with indirect coverage changes

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c1acadb...c5adf11. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.