Refactor glotfilt to use traits

I want to set aside some time to actually do this right. It's not quite fitting in my head at the moment, and I need to keep moving forward. Ultimately, what we want is to be able to have a nice way to define two two types: highpass and lowpass, which are two types that use identical data structs but slightly different methods.

I imagine it'll be pretty trivial to return and refactor. So far, this has been my experience with Rust.