Fix broken docbuild on RTD
Created by: paulromano
Our 'latest' docs on readthedocs are currently broken (can't view docs for Python API classes/functions). This is caused by
import openmc.capi at the top level of polynomial.py. I've moved that import out of the top level so that doc builds will succeed without building OpenMC first.
I've also put in a fix for some issues with openmc-plot-mesh-tally that were identified by a user recently.