Several fixes and improvements:

    - Simplify reporters so that the trajectory and restart reporters figure out
      the number of atoms and presence of the box based on the first state of
      the first frame write
    - Update the openmm simulator in ParmEd for these changes in reporter APIs
    - Prevent automatic scaling for netCDF4 package in trajectory files
    - Make sure data is flushed to disk after writing sections to files
    - add a "box" attribute to NetCDF trajectories that behaves more like the
      box attributes from other classes
    - Improve several docstrings
    - Fix bug in which velocities were doubly-scaled in NetCDF trajectories
Add tests for each of the reporters.

Test coverage is fairly extensive at this point -- the only things missing are
the energy and openmm actions in the ParmEd interpreter, the strip action for
AmoebaParm objects (which still needs to be fixed), and the parts of the ParmEd
interpreter itself (also illegal uses of the reporters are not currently
well-tested).