Add support for generating caves

[oubiwann]

This would most likely be ASCII representations of caves.

Tasks:

• Identity a good noise library
• Create a prototype app for it (see Create a noise app noise#1)
• Create a docker image for the app (see Create a Dockerfile for noise generator docker#3 & Noise generator: move from example to app docker#4)
• Convert the app into a gRPC-based service
  • Consider running a gRPC service - Consider running a gRPC service noise#2
  • Define gRPC-based execution flow - Define gRPC-based execution flow noise#3
  • Implement gRPC service - Implement gRPC service noise#4
  • Generate Go lib for noise service
• Update the CLI to generate caves - Add "caves" command cli#6
• Devise mechanism for assigning caves to geophysical location on map
  • With a pixel representing ~1000km², many cave systems could fit on a pixel
  • Subpixel indexing will be needed, following on the Hilbert curve work being done in Explore mechanisms for storing, indexing, and retrieving raster data raster#25
  • Provide a means of storing caves in the raster service's DB (see Extend storage mechanism/indexing/retrieval for supporting high-res features raster#27)
• Support image-generation of cave raster data
• Support ASCII-generation of cave raster data
• Provide a means of linking generated caves in support of creating cave systems

[oubiwann]

Of all the open source noise-generator libraries I looked at (Go, Clojure, Haskell, Rust, Java, C, and C++ libs were all considered), the Rust one was the best-designed, cleanest code, and easiest to extend (without having to fork the repo). Very well-done software design; kudos to @Razaekel.