Proper output of ddouble numbers

[mwallerb]

Right now operator<< simply prints the hi and lo part. Ideally, we want to change that so that we can simply print the number.

[nonbasketless]

I've gotten this done, I wrote a function with signature

string to_string(DDouble d, size_t nDigits = 34)

Want me to PR and you can incorporate however works best for you (such as <<)?

[nonbasketless]

Or I can just share the function, it's self contained, whatever's easier for you.

[mwallerb]

Feel free to open a PR!

[nonbasketless]

Done. You'll probably want to make it follow your style better.