Resolve "Refactor stream filters"

There are 4 new modules in this merge. The first 3: "typing", "utils", and "validate" are support modules intended to be used throughout the package. The final "segments" module is the new scientific bit - this module applies various filters to an initial stream segment network.

I'd recommend starting with the support modules - "typing" and "utils" are particularly small, so could be a good starting place. "validate" is a bit larger, but its functions are relatively straightforward.

As the user-facing module, "segments" is quite a bit more complex. Maybe save it for last.

Closes #14 (closed)