We should handle chained dependencies more carefully

[lothian]

We have a natural hierarchy of dependencies within PyCC, e.g.:

hamiltonian <-- ccwfn <-- cclambda <-- ccdensity <-- rtcc

However, the __init__ functions for objects higher in the dependency structure still require explicit passing of lower-level objects, e.g., in the rtcc object:

pycc/pycc/rt/rtcc.py

Line 60 in 34f928d

def __init__(self, ccwfn, cclambda, ccdensity, V, magnetic = False, kick = None):

This API implies some flexibility as to which ccwfn or cclambda objects the user might pass into the constructor, as though it could actually be different from that used to build the rtcc propagator, but this is clearly not the case.

I think we should change the higher-level interfaces to be more careful about exposing fragile dependencies to user whims/errors. This would also simplify the interfaces, which is always nice, but, of course, would require changing all our test cases.