NdArray: Add stream operator
python/repr.h: Add repr for PyObjects
python/numpy: Explicitly disallow None -> numpy array conversion

PyArray_FromAny silently converts None to a singleton nan,
which is not cool.
python/wrap_constructor: Allow extra arguments when creating derived classes
PropManager: Various cleanups

1. Remove confusing default arguments to add.
2. Move more stuff to .cpp.
Prop: Make make_prop fall back to default for misunderstood arrays
value/convert.h: Simplify
Worker: Various improvements

1. Make ValueProxies inherit from PropManager for C++ compatibility.
   For this purpose, PropManager now has a frozen field to enforce
   temporary immutability.
2. Restructure master/worker protocol
3. Support arbitrary control flow on the worker.
4. Fix a variety of exception-causes-hang bugs.
force: Add Force::nodes() and reformat a bit