Support for multiple numeric types

[boxtown]

Currently statrs only supports f64 but I'd like to at the minimum extend that to f32 and possibly other numeric types as well (especially for things in the statistics module). The num crate might be worth looking into but I'm hesistant about introducing the dependency when it might make it's way in to the standard library at some point.

[vks]

I don't think num will move to the standard library. In fact, it has been moved out of it.

[boxtown]

@vks I was under the impression that a float trait would make it's way into the stdlib once associated constants is implemented and stable? I'm not saying I refuse to use num just hesistant for now

[vks]

Some traits might be moved. For now you could just depend on num_traits, which is a fairly lightweight dependency.

[boxtown]

Yeah it just depends on when I get to this. Trying to port statistics::Statistics to Iterator is being a HUGE pain in my side

[boxtown]

This has been a really huge effort and pain with in my opinion not enough tangible benefits. Hate to do this but punting on this until the future if ever, and will restart v0.4.0 with other more useful targets