Started branch that put inlet dynamics in separate modules -- not done

[anardek]

Last year I started to improve the readability and testability of brie.py by putting the inlet dynamics in a separate module (similar to the AlongshoreTransporter): I named this module the InletSpinner. It is not complete. Hoping myself or @avanblunk can pick this up in the near future. Will be necessary for adding inlets to CASCADE.

This other branch kanarde-improve-AST-inlet-modules also has more documentation for the AST module (in the form of a Jupiter notebook).