Consistently use atom/expression naming in README

[maxitg]

The problem

Words expression/edge and atom/vertex are currently used interchangeably in the README. We need to standardize that and consistently use expression and atom throughout.

Possible solution

Essentially, we just need to replace all instances of "edge" -> "expression" and "vertex" -> "atom", except where edges and vertices refer to elements of graphs other than the state hypergraphs (i.e., causal or expression-event graphs).