Check all of special functions can be included under same conditions

[mborland]

CC: @PaulXiCao

This seems to run fine with all of special functions included. It looks like the other naked uses of <sstream> are in the interpolators and gradient optimizers. Those should all be outside of what you need, and are effectively isolated modules within boost.math.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.39%. Comparing base (188cccc) to head (9fbea69).

Files with missing lines	Patch %	Lines
test/git_issue_1412_pt_2.cpp	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1416      +/-   ##
===========================================
- Coverage    95.40%   95.39%   -0.01%     
===========================================
  Files          827      828       +1     
  Lines        68921    68925       +4     
===========================================
+ Hits         65752    65754       +2     
- Misses        3169     3171       +2     

Files with missing lines	Coverage Δ
test/git_issue_1412_pt_2.cpp	50.00% <50.00%> (ø)

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