Add support for using constexpr constructors where applicable

[Ozzymandia5]

ConverterParams, LabelParams and pretty much almost all of the structures in rapidcsv are lacking constexpr constructors. This is not a limititation raised by the code, so why not?

[d99kris]

Hi @Ozzymandia5 - I haven't evaluated the pros/cons of using constexpr constructors in rapidcsv. I'm open to consider using it, I'll probably just need to make sure there are no cons using it, like; debuggability, portability (ensure usage works with all C++ >= 11 incl. 23), etc.

[d99kris]

After doing some research I agree it looks like a good idea to make those constructors constexpr. Feel free to submit a PR, or otherwise I'll prepare the changes myself eventually when I find some free time.