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These are worth fixing because they're in header files that are always included
(rather than just hitting them at OpenMM compile-time). The SWIG interface now
builds without warnings on my machine.

Swigged python docstrings now include documented return values and type
information or their arguments. They are generated in numpydoc format.
Furthermore, all of the Python app layer docstrings have been changed
to numpydoc format. The filterPythonFiles.py script which helps to
generate the Doxygen Python API docs has been updated to reflect these
changes.

New advanced options OPENMM_BUILD_CPU_TESTS, OPENMM_BUILD_SERIALIZATION_TESTS,
OPENMM_BUILD_REFERENCE_TESTS were added to the cmake configuration and disabled
in the OpenCL test matrix element to increase speed. Redundant or unnecessary
apt packages removed from the travis installation.

- Remove API change for AmoebaAngleForce so it goes back to only taking degrees.
  Handle conversion via a pythonprepend (since stripUnits is done in C++ now).
- Do the same thing for TorsionTorsionForce, since angles are in degrees again
- Add AmoebaVdwForce and AmoebaWcaForce tests

angles are in degrees (unlike *every* other angle force out there), and
stripUnits automatically reduces all angles to radians if they come in with
units.

The approach here is to *slightly* change the API, so that
AmoebaAngleForce.addAngle interprets input angles <2*pi as radians, and >2*pi as
degrees. This is heuristic, but should work in every case out in the wild so
far.

I've also updated the documentation to reflect this behavior, and fixed the
units attached to the return value of AmoebaAngleForce.getAngleParameters() to
return degrees instead of radians.

o Replaced Cartesians in the reciprocal and self PME terms.
o Added non-PME spherical harmonics field generation and force evaluation routines.
o Added spherical harmonic potential evaluation, to allow removal of the Cartesian moments.