All pipeline resources are constructed at least twice

[timkurvers]

Currently all constructors for M2s, ADTs and WMOs are invoked twice, once to populate the cache, then once more to provide a fresh unique copy.

There's bound to be a better way to do this.

static load(path) {
  // ...
  return this.cache[path].then((m2) => {
    return m2.clone();
  });
}