Added more functionality for NodalData

[stepholinger]

This PR includes new types and functions for handling manipulation of NodalData. The content of this update should all work and be useful, though there are already a few ways I can think of to improve some aspects of the code (i.e. there is some unnecessary-feeling metadata that gets passed through the NodalProcessedData type that is inconvenient in some cases). Happy to meet to chat about any of this stuff!

Future updates will include FK filtering and batch correlation tools, though we should probably chat about reconciling some of that with the existing nodalcorrelation functions. I have a few other helpful functions I didn't include this time as I'd like to go through them in a bit more detail.

[tclements]

This is awesome! I'm going to transfer SeisNoise to the SeismoJulia org shortly. How about we work on this PR after we transfer over there?