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errors.

This copies the fix for the AmoebaTorsionTorsionForce over to the other two
forces that calculate angles via the inverse cosine -- specifically the in-plane
angle force and the stretch-bend force.  The TorsionTorsion fix did not
completely solve the occasional simulation blowup, although it did make it a
*lot* less frequent (it went from happening within 5 minutes to happening ~once
per day).

The forces were not correct for different uind/uinp as a result.

Non-PME OPT calculations now use a much more efficient Cartesian field algorithm for the dipole response force terms.

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.

The QI direct space energy uses the energy functional

    U = 1/4 (E_p µ_d + E_d µ_p)

while the code it replaced uses the equivalent form

    U = 1/2 E_p µ_d

Therefore the reciprocal and self terms have been adapted to use the more symmetric form.  The bad code yielded incorrect energies when PME is used AND µ_d and µ_p are different.  The forces and torques were not affected.