GitLab

* Began implementing geometric functions on points

* Started common implementation of point functions

* Completed implementation of point functions for CustomCompoundBondForce

* Implemented point functions for CustomCentroidBondForce

* Implemented point functions for CustomManyParticleForce

* Use point functions to simplify implementation of custom forces

* Removed unnecessary code

* Fixed typo

Platform specific header files get installed.  This allows plugins to be built with just an OpenMM installation, not a full source tree.

Free energy plugin added
Plugin will not run w/ Obc or GB/VI forces unless line 2004 of gpu.cpp (gpu->sim.totalNonbondOutputBuffers  = 2*gpu->sim.nonbondOutputBuffers;) is commented in -- working on removing this constraint
Also unit tests for GB/VI currently fail 

Forced computation of dot product used in computing angles to be done in double precision; acos() is very steep for angles near pi and single precison was giving disagreement w/ corresponding Gromacs values (discrepancies of the order of ~1.0)