Add free function for calculation of simple continued fraction coefficients

[mborland]

I think this solves #971 while keeping the class immutable like @NAThompson wants. Since this header requires C++17 guaranteed copy elision should make passing the data from the class to the user through the free function inexpensive. @Tomaqa I used your use case as the additional test case.

[mborland]

The CI failure is failed to clone.

[Tomaqa]

It is a way.
However, I need to convert the resulting coefficients back to a rational or floating-point number.
It can again be achieved by a free function which operates upon a vector of coefficients, but I think it is not conceptual. I just want to convert the resulting simple_continued_fraction to rational.

A possible way is to add constructor simple_continued_fraction(std::vector<> coefficients) where the coefficients will be checked whether they are valid, which should be easy to implement and inexpensive to run.
Also, std::move can be used.

If this idea is fine with you, I can implement it, but I will need to use your commits.

[mborland]

If this idea is fine with you, I can implement it, but I will need to use your commits.

Go ahead. This is on a branch in the boost.math tree so you will be able to clone it.