Floating point <-> string conversion

[Robin Williams]

I happened across this

  https://isocpp.org/blog/2020/08/cppcon-2019-floating-point-charconv-making-your-code-10x-faster-with-cpp17s

describing why it might be worth using this

  https://en.cppreference.com/w/cpp/header/charconv

when reading and writing lots of numbers as strings (at least once it becomes generally available, the talk is about the MSVC implementation, but it sounds like it's in process for g++ and clang).

  Robin

[Gary Ferland]

we had tried to do only C string conversions due to the speed hit of C++.  This says that the performance boost over C is even significant.  Probably worth looking into, especially as we large more and more atomic data files.

The current (V10) version of Chianti is about 2GB.  Version 7 that we include in our download is about 100 MB.