Replaces global pi value with std::numbers::pi_v<T>

[cugone]

Fixes #67

This replaces the global double-only pi variable with std::numbers::pi_v<T> from the <numbers> header. As a bonus it plays nice with templates and can now be represented as both a double and float depending on the template argument type instead of implicitly being cast to a float during multiplication operations.

[OneLoneCoder]

Hmmm, firstly it ruins my awesome joke :( . Secondly, I'm unconvinced it's an improvement - it's wordier and less clear. Just because the std has something, doesn't always mean it should. However I'm fair and quite malleable, convince me and others it's better and i'll merge it.

• I'm unconvinced about the casting, I would suspect that's resolved quite happily at compile time.
• I'm unconvinced it's clearer, keep in mind the audience of this library.

[cugone]

https://godbolt.org/z/7eT476ocM

I had to convince myself as well. They produce the same assembly output.

The clarity argument can be mitigated:

• It's internal, at least for now.
  • "Excessive" namespace qualification can be mitigated via using std::numbers and the aliases provided by the numbers library. It defaults to double though, hence the pi_v<T> cruft.