fix sphinx warnings

[dave-doty]

Currently there are warnings when compiling the docs with sphinx. In doc if you run make html (or make.bat html on Windows), there is this warning:

C:\Dropbox\git\ppsim-rust\python\ppsim\snapshot.py:docstring of ppsim.snapshot:1: WARNING: duplicate object description of ppsim.snapshot, other instance in api, use :no-index: for one of them
C:\Dropbox\git\ppsim-rust\python\ppsim\simulation.py:docstring of ppsim.simulation.Simulation:40: ERROR: Unexpected indentation. [docutils]
C:\Dropbox\git\ppsim-rust\python\ppsim\simulation.py:docstring of ppsim.simulation.Simulation:43: WARNING: Block quote ends without a blank line; unexpected unindent. [docutils]
C:\Dropbox\git\ppsim-rust\python\ppsim\snapshot.py:docstring of ppsim.snapshot:1: WARNING: duplicate object description of ppsim.snapshot, other instance in _autosummary/ppsim.snapshot, use :no-index: for one of them

But everything I've tried to remove the warnings breaks the inter-module reference links, e.g., ppsim.simulator.Simulator in the docstring for the crn module.