Use std::stringstream instead of snprintf() for double->string conversion

[cdunn2001]

snprintf() will use the current LC_NUMERIC locale
for converting a double to a string,
which will use a , instead of a . in some locales (e.g. de_DE).
std::stringstream allows setting the locale to "C" to always get a ..
This occurs only for that stringstream instance; no global is
altered.

[cdunn2001]

Hmmm. I think we also need:

str.setf(std::ios_base::fixed|std::ios_base::scientific, std::ios_base::floatfield);

[cdunn2001]

Hmmm. That doesn't seem to work, despite the link. Anyway, our tests pass. Call it done.