start to keep track of, like, quarter- and half-plane spectral sequences with a generic class
also want a class for triples, since we'll be using this for the A1MSS demo
removed from leftover leibniz code
quick routine to stupidly propagate differentials en masse
include a differential propagation option that asserts all undefined terms are zero (i.e., the SS is fully defined)
get A1MSS very close to working