Find degrees of freedom for non_central_f distribution

[JacobHass8]

Towards #1305. I've added functions to find parameters df1 and df2 because they are nearly identical.

All the tests are passing, but changing the initial guess for the degrees of freedom that is passed to bracket_and_solve causes most tests to fail. Thus, I'm not super confident in the current implementation. I will add more spot checks for a wider range of df1/df2 to make sure things don't break. Perhaps the initial guess should change based on if the different of p - cdf is positive or negative? For now, I'm going to leave it though.

I also had some difficulty with seeing if the function passed to bracket_and_solve was rising or falling. I ultimately found that the following worked

RealType guess = 1; // Changing this to 20, 200 causes errors! 
std::pair<RealType, RealType> ir = tools::bracket_and_solve_root(
   f, guess, RealType(2), f(guess) < 0 ? true : false, tol, max_iter, pol);

where

f_degrees_of_freedom_finder<RealType, Policy> f(x, v, nc, find_v1, p < q ? p : q, p < q ? false : true);

Essentially, if the difference between cdf-p (or is negative then f is increasing, and if it is positive then f is decreasing. I'm not entirely sure why this works though.

Could all of this rising/falling be avoided by minimizing the squared difference (cdf-p)^2? Maybe this will open up a whole new can of worms though.

[codecov]

Codecov Report

❌ Patch coverage is 96.92308% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.34%. Comparing base (a10f3c8) to head (3eb7561).

Files with missing lines	Patch %	Lines
include/boost/math/distributions/non_central_f.hpp	96.72%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1368      +/-   ##
===========================================
- Coverage    95.34%   95.34%   -0.01%     
===========================================
  Files          825      825              
  Lines        68160    68224      +64     
===========================================
+ Hits         64987    65048      +61     
- Misses        3173     3176       +3     

Files with missing lines	Coverage Δ
test/test_nc_f.cpp	100.00% <100.00%> (ø)
include/boost/math/distributions/non_central_f.hpp	89.70% <96.72%> (+1.73%)	⬆️

... and 1 file with indirect coverage changes

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